your questions regarding the game rules (improving the interpretation).

Flax is strong if you can build the birds on the first day.
There are no other viable strategies for him right now; upgrading fairies and psykers won't significantly boost him, but will diversify the game's strategies.

Flax shows good results; I calculated the win/loss ratio for each city in the Fortress Battle http://vk.com/homm3?w=wall-31997_33642, with over 60% wins, making it one of the most frequently winning cities. If you place the birds on the first day, it will be strong, and if you also give it the opportunity to upgrade to levels 1 and 6 on the second day, it will become an even stronger overpowered unit.