Also, why do Sirens have nearly as much HP as a grizzly for less than a third the cost?

Unit is seriously overtuned, its intended counters (mistrals/envoy) can't kill it quickly enough to properly counter it. You're stuck retreating and ceding the entire map to the enemy.

Unit is dumb, it's the only anti-turret unit in the game that also beats raiders.

just ban this map till you feel like a good balance has arrived.