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I usually don't focus on air units in the early game because, in my opinion, with fairies, it's more advantageous to play with melee units rather than ranged ones. The real problem is when you can't get your units up on the first day; otherwise, it's a very good strategy to build from level 1 to 6 in the city – it's cheap and effective. You can buy units on day 121, and with decent combat skills, you can defeat almost anyone.
However, the real strength of the Faction lies in having 4-5 units on day 121, supported by 20-25 Storms. This is probably the most powerful rushing strategy. Add to that the deadly mages that usually come in abundance, and you get an overpowered combination that was rightfully nerfed in the expansion.
Off-topic: Tower, Swamp, and Barbarian factions can easily take the maximum-level castles with the "Mass Slow" spell. Swamp can also take them without "Mass Slow" if the Warlock is properly developed. Tower is a bit more difficult because it relies heavily on the Naga's retaliatory attacks, but by adding the War Machine and upgrading the Golems, I think they can also take it – although this is debatable. As for Barbarians, I don't know them very well and haven't tried taking maximum-level castles on the second day without "Mass Slow," but I think they could do it if they had to. All of this is just my opinion; I won't test or analyze it further because I'm not interested