I think it depends. It can be problematic if you are playing in Impossible and you haven't pre-planned a build to beat a boss and relied heavily on Rage Centric strategies as a Viking/Warrior.
A Soothsayer/Mage can also have similar problems if he relies on winning early on without any method of mana regeneration mid battle. Oops.
I had to take fairly severe losses as a mage against the spider boss, even with the 'trolls'. Heck, I almost simply didn't win the battle because I don't do enough damage with units or with spells. (this was a huge concern of mine in Armored Princess that a Mage could never spell nuke a boss effectively as a Warrior using sheer leadership/damage to wipe out a boss in a fraction of the time).
Incidentally, I won by using Polar Bears and Ancient Bears, using the jacket that gave me -15% leadership for bears.
Trolls for consistent damage (and ultimately one of the last survivors) Jarls/Maidens for some damage control.
But then again, this only seems to be a 'problem' in Impossible mode so significant pre-planning and game experience is expected.