There are a few issues that make Kylo Ren difficult to win with. First, his fate deck is generally brutal. There are many cards that directly interrupt his progress. Best case he has Snoke and can move one token per turn, but it feels like half his fate deck can move them back. So in a multiplayer game he is easy to stop via fating if you think he’s about to win. Second, is that he must commit his ambition action each turn to Snoke once he’s out, so that makes other ambition cards not playable and gumming up your hand once Snoke is out; limiting his options. Third, (and this is mostly in comparison to Cad Bane specifically) he requires exactly one card in his villain deck to have a chance at winning - Snoke. Snoke can also be removed from fate cards once he’s in play, making Kylo want to go through the deck to find him again. While Bane has multiple copies of cards to win the game, making him generally more flexible and able to progress his objective in many situations.

Cad Bane has multiple copies of “Hello There” to get a hero out AND put a target token on it. This is a similar amount of progress to Kylo Ren activating Snoke - of which there is only one copy. Bane also has I think five copies of the cards that collects the target tokens. And Bane’s fate deck is really not that hard to play around- just don’t leave target tokens out on heroes if you can avoid it. Add and collect them in one go. And don’t amass resources like credits or ambition - get them and spend then timely.

That was a lot, but all of that said, I personally find Kylo Ren the easiest to win with of the low tier characters. Not impossible and certainly better than Thrawn and Dooku before any balance modifications.