Yes, it IS viable build. I playing in right now on Nightmare. But i didn't learn EVERYTHING in skills both talents. It isn't necessary at all. I didn't take extra poison stack on weapon for example. It has 350 for level 5 poison potion. I don't need more xD.
But still. I wouldn't recommend to play poisons on NORMAL diff, cuz you put yourself in some sort of disadvantage. U see, poison build has a low celling build. When u reach maximum level of it - 5, your poisons will attack for about 100-120 per tick. And u can't it change somehow. U can't increase your main stat and see upcoming poison damage. On Nightmare another reason to use it ok. On Nightmare armor of all mobs in game 150% of normal(they are very though), u can't penetrate it properly, and that's why poison works fine. On NORMAL you have 100% monster armor and u just don't need it at all. U will work pretty fine even better with just usual direct damage
![Улыбка :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)