Treant's Ultimate is pretty good actually. The only problem is it lacks damage. However, it doesn't much matter coz disable is important and it's enough for you carry to engage into the fight + spam Treant's leech seed and it's good to go. Also, I love picking this hero coz his ultimate can even disable heroes with magic immunity in which is very useful. We should also not forget his low cooldown rate of of his ulti and can engage in a fight in the next minute or so. Late game, most of his spells would be pretty much useless but with Treant's awesome skills he can dominate early-mid game very easily. It's already balanced that he doesn't have much damage on his ultimate because his other 3 skills are already annoying enough:
1. Nature Guise -- can even cast this on teammates to save them or for ganking (like mirana's ULTI)
-- can even cast your leech seed while invisible
2. leech seed -- can save your teammates with heal while slowing enemy hero
3. living armor -- hp regen and damage block
-- with this on you or your teammate can even dive enemy on towers

so basically I think his ulti is already more than enough to help your teammates win a clash. Also, give him force staff or blink or use his nature guise he can be a very good initiator as well. As we all know a good initiation can win clashes. His usefulness can make the team finish the game fast :tis:

Well, I liked kidding on Juggernaut with Treant's ult when he thought that he can't be stopped during his spinnings but its "damagelessness" is cheap. All in all you are not alone in your team and others have to deal damage instead of you.