The sensors in phones do drift a lot. They basically realign pretty much constantly to the local gravity vector, which is kinda bad in long turns. There was a neat little app once, that basically just showed an ADI, i put it on the footrest of a 737 and everytime we did a cruise turn, it showed the turn correctly in the beginning, and then would very rapidly align to straight and level although we were still in a turn. That was a while ago, i believe with an iPhone 6s Plus, haven't tried it on my current X (which is already kinda oldish), it might have gotten better. The IRU functions with similar sensors in consumer drones and even RC planes with envelope protections (yes, that is a thing) do work quite well. however, flight times are usually in the range of up to 20 minutes, not much longer.
That said, even the mini-EFIS standby horizons do tend to drift quite a bit, check them out after a turn of some medium duration once you are straight and level again, they usually still show (an opposite) turn for a few minutes afterwards, gradually returning to straight and level.