To get that
early turn 70 treant defender - which, btw is
incredibly fast for a non-AI player on Normal speed to have FoL by then - you have to
* spend over 2000

on a technology path that meanwhile
* forces you to rely mostly on warriors and hunters and hope
* that the 1 in 20 chances of a treant appearing (1 in 7 with a Leaf Temple)
* in an unoccupied Ancient Forest tile, the few you have
* assuming the invader doesn't avoid them anyway
* and if the treant actually appears and defeats an enemy
* his comrades run away 'til Treebeard dies ("This unit will expire in +2 turns")
It's hard to base a defense on treants; they're an occupational hazard when attacking an FoL nation with ancient forests, little more. I suppose invaders remember the treant appearances best whereas we FoL followers see the 95% of the time where no treant appears and enemies roam virtually at will pillaging cottages, farms and other improvements.
Yes, early game,
the treant (and I stress
the 'cause he is usually alone) is powerful and crushes the unwary warrior or two, esp. if they stick around in the forest, rather than lurking a turn or two at its edge until his sap dries up.
By the middle/late game, our lone, valiant treant hero, should he appear, often finds himself faced with a AI stack of heavily promoted troopers who don't have to run away for a turn or two.
Don't get me started on the myth of "The March of the Trees."
We chessplayers have a saying, "The threat is stronger than the execution" and it's very true for the treants.
(It's sounds like I'm all doom-and-gloom for the treants, but in truth: I like them. They do what they can. They're very thematic, very atmospheric. It does make an enemy think twice (before going ahead as planned

) I do think if treants were "all that" we'd have lots of Ljos players boasting of all the Clan or Sheaim or Doviello butts they'd been kickin' with a bunch of trees and some archers - it just isn't happenin' though.
And if the treant is tough in the early days, well... he should be or why bother? Besides he's still rare and alone, with the days of warriors rapidly passing...i)