Generally rainbow mages aren't very powerful. If the only mage that can find, say, water magic sites is your pretender, and he's a rainbow mage so he has some low water skill (1-2)...who's going to spend the water gems?
Do you really plan on using your rainbow god to cast level 2 water ritual spells? It's better to focus on a one magic type your nation has or maybe two you don't that will complement your existing magic well.
Unless you're Ashen empire ermor or something...then enjoy the best rainbow mage/supercombatant/lord of the undead you can create.
That's half the fun.