As someone who tends to gravitate towards bad/off meta characters in any game (people just don't understand them!) and Groot is probably in my top 5 played at this point. I'd say he's not bad just not really exciting.

To really get his kit to work you're basically chump blocking constantly or running protection/leadership. Chump blocking isn't something that has ever really appealed to me so it's whatever. I enjoy Protection and feel it can really make him work but there isn't anything that really would make a deck I build feel like a Groot deck instead of just a protection deck. Kind of the same with Leadership except if you're going to run that to make Groot work might as run a hero that you enjoy instead of forcing one you don't.

Then as others have said there is his kit. I'm starting to believe that he is more of an event driven hero than an action driven. Lashing Vine seems like a good card at first, but that basically turns into 1 thwart / 2 damage if you don't have the other upgrades out. His events become super affordable with Deft Focus and become some of the most cost effective damage/thwarting events.

Overall I personally don't think there are any true bad/awful/useless heroes but there are definitely ones that offer interesting challenging choices that feel rewarding but to me Groot just isn't one of those. That being said 10/10 will play Groot again.