Fighters and following thralls are definitely the same, here’s a list of literally all of them with their stats:
Wasn’t sure about crafters, I know that they made effort to make them the same as regular ones so I just checked how that’s looking.
Crafters are pretty much identical as well, the only difference I can see is that the Purge cooks, tanners and smelters seem to have a slightly increased crafting speed and the Purge cooks have the the recipes of the Lone Fisherman except for the spicy dried fish combo. (regular Tier 4 cooks have roughly half the amount of special recipes as a purge one).
Other than that the rest are identical to their non-purge counterparts.
Yes, because the Lone Fisherman is its own category, it is the best cook in the game 
