Legality depends on the tournament format -- Damage Control Crew can be legal in some formats and banned or restricted in others
Using Spine of Ish Sah and mana generators like Caged Sun can ensure your opponents threats are under control
Abilities that trigger when a player casts a spell (like Mirrorwing Dragon's ability itself) won't trigger
The set and the block came right after the Urza block (Urzas Saga, Urzas Legacy, Urzas Destiny), which broke standard in all sorts of ways and resulted in numerous bannings
Making a flying drake token on each of your turns, and possibly on your opponents' turns, is strong enough
Normally, I'd say that maybe this could be printed in a set focused in a format other than Standard, but delve has proven to be problematic in most larger formats