Okay, I just got all my new MMS 3rd naked bodies in today. Here are the issues as far as I can tell by the fiddling around with them I did (note, I only got tall bodies)
Neck/head The neck unit is now attached to the torso via a circular joint pinned into the torso, rather than a ball joint. It makes it so the neck is very dextrous, compared to the old bodies, but as stated earlier, has the risk of breaking.
Chest. The chest uses a wraparound breast type, which is like the chests partio and pomock uses. They have an adapter chest peice to use old shinki chest units, but these adapter peices suck, at best. The chests are loose and seem to stick out too far when using the adapter. Without the adapter peice, the old chests are unusable. This issue isn't only with wraparound chest peices, as I tried it with an Ach chestpeice, a harmony grace chestpeice and a eukrante chestpeice. The chestpeices do fit, but they're pretty awkward.
Legs. If you want to remove the old legs and replace them with an mms 2nd or below, they will work, however, the mms 3rd legs cannot fit nicely into an mms 2nd or below torso. Additionally, all leg units that utilize the shin connection point probably won't work. I know eukrante's boots don't work with it. The thigh seperation point doesn't hold older equipment peices well. This was tried with Ach's legplates. Leg units that equip onto the washer with a female connector (those washer connecters that come with all the shinkis, that can go on thigh, upper arm, or hand) will work, providing you use one of the washer/female connector units that came with the MMS 3rd. On the plus side, MMS 3rd naked bodies come with feet with toes. I like toes.
Arms. Same issue with legs. Tried attaching ach's armplates and eukrante's armplates to an mms 3rd. Didn't work so well, very loose.
Back. All the old armors that utilize the two lumbar back pegs DO NOT WORK. I attempted to use two of my armors, and the same thing happened. The screw they stuck into the upper back peg makes the back unit fit only slightly in. I suppose a figure can be displayed with a back unit like this, but any excess motion will cause the back unit to fall off.
Other than that, the MMS 3rd bodies are arguably very nice. They're made of a softer plastic that seems less rigid and breakable, The new thigh/hip joints work far better, as the rounded upper thigh section doesn't scrape against the hip area like the old shinkis used to. The breasts come with slight nipple marks (VERY slight), and they're just as posable as before. The new wraparound (over the top) chestpeices are a change from before, but still, rather nice, and are pretty okay on their own. They also come with an extra set of hands, so that they can hold swords better, or something (so, they come with the gun hand/stationary, gun hand w/wrist, sword hand w/wrist, fist, open hand). Oh yeah, and the toes. Nice touch.
Overall, the MMS 3rd fixes a lot of issues I had with previous shinki bodies, however, they are HUGELY incompatable with the old armors to the point of wondering WHAT THE FUCK. If you're buying MMS bodies to use with EX armor sets, try, try and find a body that's MMS 2nd or below. If you're making your own light armor shinkis out of extra heads and spare parts, then by all means, the MMS 3rd is a delightful addition to your shinki collection.
It's freakin' late. I'll post pics tomorrow (technically later today).