Everything.
Presentation. Not just simply graphics - which is obvious - but the Frostbite versions do a much better job of recreating the match day atmosphere. Proper league-specific scoreboards and TV presentation, more realistic fans and celebrations, etc. Up to date stadiums. Anfield doesn't have the new Main Stand even though they make reference to it in the commentary, for example.
Can't play against Friends online on the Switch version, and EA doesn't appear to be interested in fixing that. One or two features of FUT are missing in the Switch version. The Career mode is based on the same old career mode we've had for a while now, though from what I've read/played, the new version isn't exactly a huge leap forward anyway to be fair. Of course, no The Journey if you care about that.
Gameplay just feels better to me on PC as well. The game feels different this year for once. Defending on Switch can be easily achieved by taking control of a player and just chasing after the ball carrier and eventually tackling him with standing tackle or by running into the ball. This is much more difficult in the other versions to do, and you have to make more use of positioning and blocking passing lanes, and trying to contain the attack. Much more realistic.
The amount of throughballs that get cut out in the Switch version feels too over the top to me and a few others but that may be a matter of preference. The game feels a bit slower on the PC version but passing/throughballs is easier and crossing is improved from the norm. This is missing from the Switch version. It also feels like you have to hold down the shoot button much longer on the Switch version as well.
If you want to buy the Switch version or buy it for someone, then sure, go ahead. It's a good game. But for me, having played 'both' iterations, I can't recommend the Switch version over the other (again, unless portability is super important to you or whoever you're buying it for).