Going to make posts on every set of custom moves in the game, starting with Ness, I don't read japanese so I don't know the official names and it's hard to test if they have extra properties like shieldbreaking because of the barebones training mode, but anyway:
Neutral Special 1 (PK Flash)
Ness' usual NSpecial, has to travel a set distance vertically before it starts moving horizontally, can be detonated as early as about half Ness' height above him or whenever it falls to the same height that Ness used it, not much range or height to it.
Does around 8% at it's minimum charge and 35% at max charge. Has decent KO power, but the very slow nature of the move and lack of horizontal control means it's also not very useful at all.
The move also puts you into special fall still despite it providing no benefits as a recovery move whatsoever.
Neutral Special 2 (Rising PK Flash)
Can be detonated practically instantly, it's the opposite of the default PK Flash in that the horizontal distance it covers is negligible but it will constantly move upwards instead of for a set period, will go from the bottom of Battlefield right to the top of the screen.
Does 5% at minimum charge and only 23% at max charge, the blast radius is about twice the size of the default NSpecial though.
The large blast radius and high vertical distance means it's quite useful as a move for getting players falling to the stage.
Move still puts you into special fall.
Neutral Special 3 (PK Freeze)
Looks and moves exactly the same as the PK Flash, except it's blue and freezes whoever it hits based on the amount of damage they have.
Does 9% damage, 2 second freeze at minimum charge and 18% damage, 4 second freeze at maximum charge, though I'm assuming the freeze times should be lower when used on real people (if it ever actually hits them) seing as you can probably escape it by pressing buttons. Knockback is massively reduced.
Again, special fall, don't ask me why because nobody knows.
Side Special 1 (PK Fire β
Travels horizontally on ground, diagonally in air, if it hits a person it spawns a flame pillar, if it misses, hits the ground or hits a shield, it does nothing.
The flame pillar hits about 9-10 times for a total of around 20% but it is DIable and you can escape it having only taking 5 damage.
There are a lot of startup frames, enough for anyone with a reflector to use it on reaction or just dodge it by rolling towards you, and a lot of endlag on the move.
Do not use it in the air if the other person is grounded, do not use it if the other person is approaching you and you're standing still, you're going to get punished for it.
Either use the move as an edgeguard tool to try and get them trapped long enough for you to dair them or pivot cancel it when grounded, and even then the pivot canceled PK fire is still pretty unsafe because there's still enough time to dodge it.
Side Special 2 (PK Fire Ω
Travels the same distance, moves horizontally when grounded and diagonally when airborne.
Creates a flame pillar that does more damage and lasts longer but the individual hits are more spaced out and they have higher knockback, so it's much easier to DI out of it.
Will do about 40% damage if you hit someone with the entire move, but that's never going to happen.
The startup frames are exactly the same but the endlag is almost one and a half seconds.
The move sounds like a good idea on paper but the way they've implemented it means it's objectively worse than the default SSpecial, don't use this.
Side Special 3 (PK Fire α
Seems to go slightly further than the default PK fire and will always travel horizontally.
Spawns a flame pillar that will only last for 2 hits that do 1% and then 8%, the knockback on the second hit is higher and knocks them away from you, which means it's a good keepaway tool. Has exactly the same startup and endlag as default PK Fire from what I can tell.
The increased travel distance means that you can pivot cancel it from further away, make it a safer option, it doesn't have the same potential for combo setups as PK fire 1, but there aren't many combo options off of a PK Fire in this game other than 1 more move or a grab.
Up Special 1 (PK Thunder β
Ness' usual PK thunder, moves relatively fast, does 8% damage and his higher knockback than in previous games.
The thunder will disappear and put you into special fall if you hit another player or some other surface, though it does appear to have immunity to destruction on the first few frames as it will go through people at first.
Hitting yourself with it will cause you to fly in the opposite direction, travels about the same distance as it always has, damage and knockback decreases as you travel, starting from 25% damage and very high knockback to 20% with very low knockback.
Up Special 2 (Lucas' PK Thunder)
The thunder this time travels slower but can go through enemies, the head of the thunder has pretty high knockback to prevent you from hitting multiple times, but the tail hitbox does less knockback and damage, so if you manage to hit them with that, you can trap them inside the thunder and do around 20-30% damage, otherwise it does 7-8% damage.
When you hit yourself with it, you seem to go further than the default PK Thunder and it traps other players inside your hitbox, doing 1% damage a hit until the final hit which does 10% and has increased knockback, which is less than default PK Thunder.
Up Special 3 (PK Thunder Ω
Travels slower, has a larger hitbox and does 12% damage and increased knockback on hit. The move isn't piercing so it won't go through other players.
When you hit yourself with the move, you travel about half the distance of default PK thunder, but it has a larger hitbox, does 30% damage and has some of the highest knocback in the game (I killed bowser at 60% with it). Knockback decreases as the move goes on, but damage stays uniform throughout.
If you think you're not going to have a problem recovering use this, otherwise it's probably better to use Lucas' PK Thunder.
Down Special 1 (PSI Magnet)
Same as it always is, heals you when it abosrbs energy projectiles based on the amount of damage the projectile would have done, pushes people away very slightly when the move ends, has no real applications if you're fighting someone without an energy projectile.
Down Special 2 (Explosive PSI Magnet)
Has no projectile absorb properties whatsoever by the looks of things, whilst held it will suck people in towards you from quite a large distance (y'know, just incase you wanted that badly spaced aerial from the other player to hit you instead of miss) and when the button is released, does a small explosion that does 10% damage. Knockback is equivalent to his Ftilt so it's about average, the damage and knockback are unaffected by the length of time you held the button.
You can't explode instantly and the endlag seems to be less, probably ends up being the most useful version of PSI magnet if you're playing against someone without energy projectiles.
Down Special 3 (Lucas' PSI Magnet)
For this move, Ness holds the magnet out in front of him, has higher ending lag and does 9% damage if the other player's hitbox is overlapping the center of the Magnet hitbox when you release the button, they can be inside it without touching the middle and not get hurt.
The higher endlag makes it easier to punish you if you read the wrong move or something and that combined with the fact that the magnet is in front of you makes you vulnerable to crossups, but I think it's a better option than the default if the other player has an energy projectile, otherwise I'd just go with the second DSpecial.
Final Thoughts
I think there are only 2 customs that I'd say you should use over the others, Side Special 3 and Neutral Special 2, the default and other variation of the move are just useless compared to these two.
For the other two moves it really depends on what the other person has picked, using the Green PK Thunder will give you the edge against slower, heavier characters, allowing you to go toe-to-toe with them in terms of damage and knockback potential whereas you're going to want to use Lucas' PK thunder if you're playing against a fast characters that's going to be trying to jump off the stage and edgeguard you.
For PSI Magnet it really just depends on whether the other person has an absorb-able energy projectile or not, but whichever version you end up using, you should probably refrain from using the move in actual play 90% of the time, it has very few applications.
And the annoying thing is that none of this matters when you play online for the most part! Have fun not being able to use straight up better versions of moves on a character that has very bad defaults.
My personal loadout for the character (never thought I'd be using the word loadout in Smash) is 2322, though I change it to 2332 against heavy characters sometimes.
So that's that then, it actually takes a while to unlock all custom moves for a character due to the complete random nature of it and the fact that you can get duplicates, but I'm going to keep plodding on. Is there any particular character you guys want me to go into detail next? I was planning on doing Olimar.