ToME: the Tales of Maj'Eyal

In endgame one point in it gives ~100 damage, 5 points ~200 damage.
Judging by description of Feedback resource where lvl 50 character will get 100 feedback for losing 20% of hp (btw is it hp or hp+psi?), and Backlash description

you'll inflict mind damage equal to the Feedback gained from the attack or *numbers*, whichever is lower

it looks like in order to make Backlash worth 5 points character need to lose 40% hp per turn, otherwise Backlash simply won't do damage.
Also, I thoroughly checked log and didn't find any indication of it working - seems it has no indication in log, and only one case when it could be damage from Backlash - which mean it's awfully hard to trigger it for solipsist.

So, how bad is this skill? Maybe I don't understand something?

It’s a free passive attack that (I think) triggers on mind hit effects. Good at 1 point if you have the category but definitely not worth more, and if you’re just playing solipsist probably not worth a category point. On adventurer though, the whole tree including backlash is great with madness (I think from cursed/punishments) for crowd control and reduced mind resist