Both are fairly detrimental to the archetypes they don't fit.
No, not really. A thief can still be a thief as a werewolf. They may not be able to sneak around and stuff while in beast form, but they can still sneak and steal just fine outside of beast form. And the beast form buffs would make a thief handle better for melee situations than without it (say as a last-ditch effort). And a mage can still be a mage as a vampire. He keeps all his magic, and can do most stuff as normal. The magicka drain isn't particularly an issue as long as he keeps fed, and the lack of magicka regen was actually a valid playstyle in Oblivion and Morrowind.
Can a warrior still be a warrior as a lich? No, as his body is weakened and would need to rely on magic. Can a thief still be a thief as a lich? No, because he's a rotting pile of flesh and bones, and can't really work very stealthily.
Vampires are weak to fire, plus they lose all regeneration of and take penalties to health, magicka, and stamina in the sun. Those are serious penalties to deal with.
Everything except the stamina regen is how it worked by default in Morrowind, and stamina is not too important for mages. In Oblivion you didn't have health regen, and you could play without magicka regen using the Atronach sign. And you can still get the regen as a vampire in Skyrim by simply staying out of the sun. The drain penalties can be reduced by keeping fed, and can also be avoided by staying out of the sun.
Although werewolves and vampires are geared to warriors and thieves respectively, and would benefit their respective archetype most, they are still viable options for all character types (which is important since you're not restricted to the main archetypes). A form geared to mages would similarly need to be a viable option for all character types even if it would benefit mage-types most.