Tesla has nearly 1M cars on the road with phone as the key capability, haven't heard too many issues, beyond the obvious "upgrade to a 2018 or newer phone".
My 7.5 year old Tesla Model S phone app is a bit more clunky than the new Model 3/Y phone as a key feature, but it's one click to open, and one to start.
The Model 3/Y is just "have the phone on you to unlock and start", so that's nicer for sure.
