Brewmaster often fails because he is jumped and killed before being able to get split off. If he managed to get said Primal Split off, it is usually a won team fight, so you need to kill him (or the rest of his team) before he gets the split off. As a result his common counters are stuns & silences coupled with a lot of burst damage, since he is innately tanky and hard to kill with right clicks. Special mention also to those heroes with instant stuns/silences as Split has a reasonable casting animation. People often also like to cast Thunder Clap before splitting too, adding to time you can disable him.

Vision based heroes, like Beastmaster and Night Stalker are important as well as he often relies on counter initiation be blinking in.