Black Dragons

And this immunity to magic is also a drawback.

I doubt this can be considered a drawback for a Warlock. Think about it – there's no dark magic in the guild, and there's no light magic either, meaning no resurrection or vampirism. True, it might be possible to protect against summoning with a Celestial Shield, which is certainly bad, but there's also the Phoenix, and it's just as good for Warlocks as it is for Wizards or Necromancers. Plus, these creatures can't be controlled, and divine magic doesn't affect them. So, I think there are more advantages than disadvantages here.