Nihilus really was designed just to piss everyone off. He raises cooldowns, has regenerating protection, insta kills, protection bypass on all abilities, can summon, can use summons for more protection, and steals buffs. Did I miss anything?
He's got way too much going on compared to the other two, by far. I find Sion and Traya without Nihilus to be interesting and somewhat fun. Sions pain mechanic is actually awesome, risk vs reward kind of gameplay. Traya is just unique in that you have to manage her bonds of weakness. There's isn't a single thing fun about facing Nihilus, just multiple layers of overdesigned crap meant to annoy players.