Failed? He tried to drain the Exile and due to Exile's unique connection to the force, he lost a lot of his powers. He didnt try it again after and since he was already weakened, he lost the fight too. This is no assumption by the way.
Ok, I admit that I assume as well that he would naturally try to drain anything in the vicinity due to hunger including Visas and Mandalorian. I guess he was dumb not to do it.
Funny thing some time ago I checked two different sources. In one it states that Nihilus tried to drain Exile, in another - that he lost due to being weakened by Visas breaking the bond. Also, player has the option to simply beat him up without offering to be drained or breaking bond. So how we decide, which tree pass is canon?
Katarr? Kreia? Everyone else that he ever encountered and drained?
Katarr got destroyed/blasted into ruin, not drained.
Kreia got Force blasted into the wall, not drained.
There is no one else worthy of note.
Nihilus' drain is very different from that.
Or it's just a contradiction with the rest of SW universe, these kind of contradictions are quite common. There actually is concept of draining life Force outside Kotor universe but Kreia contradicted it by describing it as severing of the Force and agony that it causes.
Regardless, you agree that several characters in the mythos can stop him.
Imho Force drain is just another technique with which he can overpower weaker characters like it is done with TK or lightning but it is not a hax technique, there is simply no proof of that.