These are things I've immediately noticed with Swamp Thing on my literal first day of playing the character.
Speed: This is pretty obvious. Everyone has already touched on it, so yeah.
Startup: On some moves it's utter trash. 35 frames for both his f3 AND b3? Makes them useless in certain situations, most notably for background bounces. I can not for the life of me hit either of them after a background bounce.
Hitboxes on certain gear(clubs): I got an Epic Ezra's Razor and started using it despite it being maybe a fourth smaller most of his other clubs. I was naive and thought that it wouldn't really matter but, nope, I can swear it decreases the hitbox on his club, making his already somewhat difficult f233, f23xxdb3, etc. meterless combo basically impossible.
Swamp Thing is the one character that I can right now say ABSOLUTELY NEEDS buffs. I don't want to see him buffed into Top Tier status, I just want him to be actually viable because he really does not come across as it unless it's Decay playing him.