I'm unsure what to think about Cove after seeing a video by Gigel the Skeleton, where 100 week growth battle between Castle and Cove took place; cove won.
Let me start of by noticing the cons of the Cove town:
3 creatures are useless during sieges (however, existence of cannon can make them useful more easily than for other towns, maybe except Stronghold)
Okeanides and Birds die soon and have quite small growth (however, they have huge damage)
Mage guild is up to level 4
The special building is Grotto (usually worse than, say, artifact merchant)
Now it seems that Cove's army can defeat most of the other armies (perhaps even all of them) when they have same amount of week growths, on top of that their seventh creature is the strongest 7th level creature (unless you count black dragon's magic immunity as more powerful), losing only with Archdevils and only if during fight with Archdevils terrain blocks Hasspids.
Cove has quite a good grail (basically neverending Prayer on all Cove creatures) and only Pirate upgrades are ridicolously expensive, however they are shooting Mighty Gorgons.
What do you think? Is cove OP? Why not?
Edit: I may have not stressed this enough, but I did not know whether Cove is strong or not, as I had inconclusive evidence. Thanks to all of you for providing me detailed answers, I agree now, Cove is balanced as it has both pros and cons; being highly offensive, it lacks useful defence.