I would sooner play through FF1 with all Black Mages than I would with all Thiefs or White Mages. I'd rank 4-ofs (this is assuming class change, I won't bother separating them for now)...

1. All Fighters
2. All Red Mages
(big increase in difficulty after this)
3. All Black Mages
4. All Black Belts
5. All White Mages
6. All Thiefs

However, I don't think the difficulty of any of these should be judged by Dawn of Simplicity. That version just pampers you way too much.