I think BMW has had a continual problem with their rear floors holding together. I'm sure the E46 class action lawsuit stung a bit. Paying to fix a bunch of old cars of the 2nd or 3rd owner...who BMW probably made no money off of. You also have to look at the increase in HP amd vehicle weights over the years. The F10 M5 puts down 500+ hp & torque and weighs about 4250 lbs! I'm thinking that BMW figured rubber would wear out pretty fast. So stepping up to solid bushings make sense.
The whole PIA about this is that its hard to check the subframe bushings if you can even check them without removing the subframe. On my E46 I took the approach which is easier to replace? diff bushings or subframe bushings? So I've went with poly subframe bushings (should have went solid) and OEM diff bushings. At least I can see the diff bushings and I don't have to drop and disassemble the entire rear end to replace them.
Of course, this discussion is pointless for someone who doesn't plan on keeping their car. Me? I'm installing the solid subframe mounts when I'm out of warranty.
2015 F34 328iX GT Glacier Silver
2011 E90 M3 Monte Carlo Blue
2004 E46 M3 Imola Red