The free version of Google Analytics gets capped at a certain volume of traffic whereas Firebase doesn't. That's definitely one motivating factor to make the switch.
I'll be interested to check it out when they get around to introducing Real-Time user stats to firebase. That's a pretty crucial component of analytics which I have open all the time.
Google contacted me to switch to firebase from analytics. Seems like they're putting a lot of effort into re-launching the firebase brand this year.