I'll comment more on the side of HvsV since that is where I have a larger focus.
Vader is definitely OP. He isn't nearly as OP as Boba Fett, but yes he's definitely the second strongest character in the game for heroes vs villains thanks to his ability to crowd control an entire team, output absurd amounts of damage, and by being the most tanky character in the game while having adequate mobility to out maneuver non-force users. Once you figure out how to manipulate your primary target to make him fall straight into your light saber at the end of the choke you'll have figured out how to win any 1 on 1 duel without taking a point of damage. Even without this nice trick he can just out-stat any other character in melee range, sporting the highest health and highest melee damage with focused rage. With the right cards he's pretty much guaranteed to come out on top in a melee engagement.
Chewbacca on the other hand is not OP. When Overwatch first came out lots of people complained about Bastion, but at the pro level Bastion was almost never played because he wasn't strong enough. Chewbacca is the Bastion of Heroes vs Villains. Chewbacca's lack of mobility and lack of damage outside of his ultimate cool down make him very easy to deal with. You have to consider the objective of heroes vs villains. It isn't a death match type of format. You have to kill a specific target and defend a specific target. And that is Chewy's downfall. Chewy can't catch anyone should that person decide to evade Chewy. All the top tier dark side heroes have improved jump and most of them have enhanced run speeds. Chewy has neither. If Chewy ever kills the target it is because that target played foolishly. Chewy's inability to output damage outside of his overload ability forces him to commit a huge portion of his offensive power to an engagement and then is left helpless for a decent duration until he gets his cool down back. That makes Chewy a less than optimal defender because the attacker only has to win once, while the defender has to not lose all the time. Chewy's sound effect ques are super loud and obnoxious. Just listen for the wookie cry as the attacker and immediately disengage if he has you in his sights and then come back with nearly free reign when that cool down ends should he be playing defense. Chewbacca also lacks the precision that Bossk and Iden have or the massive area of effect and gross power of Boba Fett's rocket barrage. Chewbacca's lack of mobility makes it so that he cannot control the engagement. He has to be reactionary. That's why he does have a powerful stun and can output such strong burst damage. But he can not force an engagement or escape one. If Chewy needs to die, you can very easily put him in a 2v1 situation. And Chewy can not win a 2v1 if the 2 play it correctly. Unlike almost every force user, Chewy can not escape a 2v1 situation. Not to mention that he gets absolutely manhandled by force powers.
Honestly the dark side is so ridiculously advantaged at the moment that I think their own hubris is Chewy's biggest ally right now. People are just used to winning as dark side without any semblance of strategy and very little skill. Chewy can actually punish people who are bad. I suppose that makes bad players upset.
