The one time I had a compass failure with my P4 (original) was early on in my flying, and even though I knew better, I took off from the roof of my car. Everything was fine at takeoff, but then it went bad once in flight with multiple errors indicating a compass issue and then forced ATTI mode.
I did have full control over the copter, but the wind did too, and this was my first time being forced into ATTI mode, and thankfully, I was able to get it down into the grass unscathed before it hit a tree.
This incident is the reason why I recommend practicing in ATTI mode on a regular basis. It should not be something to fear, especially if forced into it for any number of reasons that this can occur. In my experience, the copter will fly just fine without compass inputs, but it really will not know where it is in the sense of both location and the direction it is going. It becomes completely blind and unaware.