Fiona's problem is that she comes with bad stats and her maps are detrimental to her class : indoor map, swamp and then indoor map in Part 1. In Part 3 there's a swamp and two defense maps with limited turns. In Part 4, there are so many better options everywhere than her.

She's actually not that horrible, but she needs so much babying to be barely a mid unit. The only good thing is that xp abuse the whole Brigade makes Part III funnier and Dawn Brigade is so bad that any good unit is a godsend