After having just played Serious Sam 2nd encounter, I agree as far as your views about the boss having to fit the style of the game go; all you do is shoot, and wow, all three bosses can be killed by holding down the left mouse button and occasionally collecting more ammo. Those are fun bosses!
Now, compare that to Vampires The Masquerade - Bloodlines, and what do you have? I decided to join the Chinese vampires on my first walkthrough as a melee/sneak focused character, and then you have to fight off that giant bat-thing? Hell, talk about unfair...
A more mixed example I can think of would be KotOR 2: Nihilius, walking necrosis, and Treya being the main bosses in the game. While there were some unfair fights, at least two of these bosses were well balanced. Nihilius could be easily defeated with a good party, walking necrosis by using a bit of talking in addition to offense, and Treya, well...she sucked. But 2/3 isn't bad.