I think you're all severely underestimating Jace.
Jace isn't the worst, he's the best PW of the bunch on the back of needing no work to flip and being 2 mana that can even block mana critters.
Jace is basically autoflip as soon as you untap with it, and then you essentially have a two mana planeswalker that has a +3 that loot but you can use only once, then you have the flipwalker that has a totally useless ultimate but a usable -3 and +1. 2 mana cards advantage engines are nothing to scoff at.
Think of it this other way: it's a looter that first turn loot, second turn draw, then it's half a diminish, then it draw another card and die.
Liliana is a lot better in a vacuum, but it's a 3 mana creature that has no way to flip itself unless with another card (or a second Liliana lol). It's nowhere as reliable.
Nissa, Gideon and Chandra are just bad. Nissa need too many lands to do anything, Gideon can't attack into anything pretty much, and Chandra is borderline but too hard to flip (since you won't be attacking with a 2/2 on turn 4 and casting 2 red spells the same turn is not easy in burn after T4)
Also Pia Nalaar isn't a 2$ rare. It will see constructed play.
You may quote me and shame me in a month. Actually we should do a shame topic for MtG speculators