About BLU

[Canadane]

Because while whm may not have regen (which is really bad to rely on anyway), blu could ultimately have a single attack and that's it. Even at max level.

[Garnix]

Because while whm may not have regen (which is really bad to rely on anyway), blu could ultimately have a single attack and that's it. Even at max level.

And the other way around too: even synced, BLU still have access to all their spells, which could make them extremely powerful compared to other DPS in low level dungeons.

Regarding PotD, the way BLU unlock spells is probably one of the reasons why they did not allow it. (either that, or the same excuse they used for not allowing BLU in Ultimate : "It would be unfair for those who took the challenge with standard jobs", which I think is not a good excuse.)
Other jobs going in PotD start at level 1, and learn their spells as they level up and progress through each floor.
They would need to change the system a little bit for BLU to fit in there.
They could do a BLU-specific version of PotD, where you would learn the spells from the monsters? Or they could just give you access to your whole spellbook from level 1.
I'd be fine either way, but I would like them to consider doing something about it indeed.