So, after initially trying on launch and being disappointed in him, I recently went back to Groot. Whatever happened before, must have been a healer issue. Because when I'm getting proper heals, Goort seems a little OP.
Not only he tanks damage from the enemy team, more importantly he breaks enemy healer's LoS. So he also acts like Ana of this game, disabling the enemy tanks' heals. Since pretty much all the healers in this game need LoS to their patients.
And his ult is instant, and while it doesn't stun the enemies, the root disables their movement abilities. And also, apparently the ults, because ults in this game have movement ability properties(?), I dunno. While it can't delete enemies with big dick damage, the very fact that it's instant can be a bigger advantage. Since it wouldn't let enemy healers pop their ults. Unlike, say, if you'd shoot a slow moving projectile ult at them.