Hydras are also easy to kill — however, hitting five at once without retaliation is a powerful thing — it's a pity they were nerfed in 5.
Titans — shooting is, of course, good, and a shot of such power is even better, but in 3 it's easy to reach them and block them, which reduces their value.
Archangels — flight + resurrection — a powerful thing and very useful as well.
Phoenixes — they are priceless — the quantity and quality of the fire shield and resurrection — although the resurrection is weaker compared to 5. That's a slight drawback.
Wraith Dragons — a bit weak, and aging doesn't help much.
Arch Devils — teleport and no retaliation — a powerful combination, but they fall short on health — why on earth did they nerf them so much in 5((:(