I have build my own gimbal and everything seems to work pretty well. The cam is stable. Unfortunately, I could not really fine tune it because there is one major issue. While standing still, the ROLL motor (very) slowly turns and never stops (at 45degrees it does, I think there is a limit)
Yes, IMU calibration is typical reason for this, the other one is RC calibration. or that unused analog channels are configured in RC, always set RC to none, if nothing is connected to that channel.