The subframes crack due to careless refitting of the engine/engine mounts. The studs (on the bottom of the mounts) that drop through the holes in the subframe can catch the reinforcing plates that are spot welded to the underside of the hole, and breaks them off. Once these are damaged or missing it's only a matter of time before the engine rips a hole in the subframe.
I have never seen one damage the subframe with the reinforcing plates still in place and intact.
When refitting the engine you need to engage the primary shaft, pull the bellhousing and engine together with a couple of bolts, then lower the engine slowly and make sure you locate the engine mount studs in the holes, as the lump goes down and the studs go through the holes give the mounts a wiggle every 5mm or so and make sure they havent gone tight and the threads aren't grating on the reinforcing plate.
Before you refit an engine, get a small grinder with a flap disc on it, and chamfer a 1 to 2mm, 45° angle on the front of the primary shaft nose. This makes it an absolutely ridiculous amount easier to locate and engage the primary shaft in the clutch splines. I've been doing this for many years now, it makes such a difference.