Removing a BGA chip is almost always best done with properly profiled hot air rework. When the solder balls melt. Lift the chip off. However, the chips the OP is talking about have been flooded aroundand underneath with epoxy (flooded around solder balls). By the time the heat has broken down the epoxy, the chips goosed.

Milling the chip away is totally doable. There are plenty of videos out there of it being done WITH CHINESE CNC MACHINES. It stands to reason that the process is virtually identical milling through the pcb from the other side (to salvage a working chip).

Will deffo want dust extracted milling either direction.