Eh, MnZn and NiZn are both used. Depends. NiZn "wideband" I would say are the most common formulations (e.g., Fair-Rite #43, 44).
The loss isn't a problem at most power switching frequencies (< 500kHz).
What you do have to beware of is magnetization. They saturate readily from DC, so they are only suitable in AC transformers and full wave converters. You won't have good performance (or any at all), even from a quite oversized core, in any method that requires DC (half wave forward converter, flyback, etc.).
Tim