Barkskin subtree sounds extremely underwhelming

The effect is hard to place, but it seems to have 5+ stacks and builds back up if you haven't been hit for like 5 seconds.

That's not the problem, as I haven't tried it. The problem is the conditional Ailment avoidance. I don't see this part of the ascendancy being taken ever, honestly, and Barskin has other avenues that could be interesting to modify.

Unless it's very powerfull and/or easy on the mana res most players won't have the ressources to reserve more mana for it anyway is my guess.
Can't imagine not running Determination in favor of this, although we will see.
That tree lives and dies with the power of the tinctures and flask buffs and the rest is just where do i put my last 2 points is my guess.

The whole Warden tree seems to be like that: You pay a notable to get something which will cost a lot to use (flask slot, gem slots, reservation). If the others are as bad/niche as the gem slot loss one then its not going to be popular...
And the ailment one is likely close to useless I guess although no extra cost.

Seems like something you might use when you're armor capped.
I don't see it replacing Grace since you can get flat 40-50% DR and it works fine.

There's certainly better defense layering once you add items into the mix.

Yeah, lesson of the seasons is probably most useless ascendancy point you can take.

The conditional ailment avoidance made me laugh as soon as I saw it, I feel like it was a 4pt as well.

Barkskin itself sounds interesting no problems with that but a 4pt'er related to it needs to be vastly stronger than what is presented.

Without the numbers, it's impossible to tell. There does, however, seem to be an interesting interaction with Trauma and Iron Reflexes. You end up damaging yourself, stripping the bark giving yourself evasion instead of armour, which you convert back to armour at most likely beneficial rate, and you get to keep the maxed ailment avoidance at no further cost.