An economic event, Mass Heal mainly can give you a lot of value, but it requires a lot of setup, often more than it's worth it. 

Sera has the tools to keep her units alive with smart use of Citadel Knights and Guardian Spirits.  You can keep an army of wounded units just waiting for Mass Heal to refresh them all.  If you can heal 2 units for 2 life each, then this event is worth it.  Sometimes you can set it up so you heal 4 or even 5 units, and in those cases, it can prove to be a massive economic advantage.
However, players will do their best to focus fire to kill your units, so it will be hard to get too much value with Mass Heal, so it’s not easy to build up to and for that reason you may not want to hold his event in hand unless you see the game is developing down a path where you can do a big, impactful Mass Heal.