I'm going to be one of those people who wait until 12.1 or 12.2 this time. I upgraded right away with iOS 11 and had my phone constantly lock up on me or randomly restart, that was not a great experience. Especially when you're renting a car that can only be unlocked through a phone app and your phone is dead for 30+ minutes.
In general, I'd agree with you. I still remember how a newer iOS release brought my iPhone 4 to near-unusability because of slowness. But this release is different (really). They really got it to be faster and better.
Reviews are heralding this version as being quite stable, and my experience since the early betas matches this. I encountered a handful of little quirks, but no crashes or restarts.
Maven by GM is like this. It's a little bit clumsy at times[1], but it does the job for hourly rentals (the primary purpose of the app). The cars have Android/iOS chargers in them. As far as I can tell, you would be in quite a bit of trouble if you ran out of battery.
[1] If the internet connection is weak, it will attempt to update trip information before letting you unlock the car. You're better off turning on airplane mode.
Most of the car share companies operate this way near me. It's not "exclusively via app", but most people rely on the app and NFC to unlock/start and lock/end their trips for convenience.
That said ... you are in a car that can charge your phone too.
