It's "usable" in Nemesis, but the CRs and logic are not tailored to that league (or any hardcore league for that matter). One wrong rare affix or a nasty pack and you'd be dead before you could even chicken with how this game works. If you don't have good gear or don't watch the bot to make sure it doesn't do anything silly, then chances are you'll die. Hardcore botting requires some time investment and baby sitting in this game. People do bot Nemesis though, and some find ways to be successful for whatever they do, but that requires having a good feel for the game and knowing the current limitations of the bot. You'd want to bot the easy areas and farm up through them and do the harder areas yourself.
There is a
3 day trial available for purchase to see if you want to bot in this game or not. Path of Exile is not anything like Diablo 3, so if you're used to botting there or a MMO like WoW, things work a lot differently here.
Feel free to check out the
Exilebuddy Release/Beta Revision Guide as well as the other stickies in the forums to see if this is the right bot for you or not. If you have any other questions, feel free to ask.