Isolated tab can, with overload that leads to overheating, short out to the drain, probably around the time drain, gate and source all bond into a single conductor, just about a millisecond before the smoke escapes at high velocity. Best o make sure this will not occur, or to at least add in extra protection physically, so an insulator will probably be a good idea, or an isolated heatsink, or connected to source instead for the low side, so a failure will cause a fuse to blow to provide protection. Not a good idea to have it connected to ungrounded metal heatsink at all, so for class II use you would always add in the extra isolation of a thermal pad and insulating clamp down hardware.