Other reason Treachery is ''better'' (not better, just different) than Forty is when some seal boss/lister is both light/phys inmune. The Venom proc on treachery counters that

And well, I meant that with this setup (Treachery, Vampgaze Cham, and Infy in GT) the merc survives in Ubers perfectly, so, for rest of PVM, it's a good combo: CBF, DR%, RESIS, CURSE REDUCTION, FAST ATTACK SPEED (leech), Venom source for inmunities, fhr, etc. Damage and Defense may be lower in the screen, but tbh, defense is useless in PVM on a merc most of the time...
Giant Tresher, that's it.