INAV suffers from horizon drift since day one. It's not that Betaflight does not suffer from it (both Betaflight and INAV originate from the same parent: Cleanflight). In Betaflight it just does not matter at all. In INAV it's a real-life problem tho.
So far, we somehow managed to mitigate horizon drift on airplanes but it's not really fixing the problem. It's there and it's irritating as hell.
Last month I published a post that describes my intent to use the external board as a secondary IMU for INAV. It is "of the shelf" Bosch BNO055. Today I finally put it into a test and to be honest, the results are very optimistic!
More about INAV horizon drift: