Fear isn't an issue anymore for fearing NPCs, it was recently fixed with some nice code
Though, I think sometimes players being feared can cause them to get stuck in things or fall through the world.
The dragons mentioned probably had fear removed from their spell tables before fear was fixed and it never got added back to them. You can test it on players and see how well it works and if you think it is ok, then you shouldn't have any problems just re-adding it to the dragons.