You're right. It does depend on which instruments are being used. Midi pads or strings will just ring on and on and on. Pianos and bass are fine as real tracks but not in midi.

I don't know about the 8 bar thing. I can't even get good results with 2 or 4 bars. If I write a held chord in a "generated" ending, it appears that BIAB just ignores it and plays the regular ending and not a held chord.

The most user friendly way, to me, would be to just have the last chord ring out longer no matter what "generated" ending is used for a given style.