So regarding the Advanced Keygroups, TBH I haven't had a chance to look into this in any detail at all but to clarify what is being discussed:
A 'keygroup track' can have multiple keygroups.
In legacy, each individual keygroup within a track can have it's own filter, envelope, LFO settings etc
In advanced mode, filter, LFO, envelopes are seemingly 'global' to the entire keygroup track as a whole.
So might seem restrictive in some cases, obvious one would be where I say, have keygroup:1 as a sawtooth, keygroup 2 as a vinyl noise sample. I would likely want the noise to have it's own independent filter, perhaps a fixed high pass, while the sawtooth would have say, a low pass or whatever. And I'd want independent control over each keygroup's envelope(s), for example the sawtooth could be a long release while the noise is set to create an short noise burst.
In legacy this was done directly editing each individual keygroup, now I'm not entirely sure how I'd have the noise with it's own independent adjustable envelope setting in advanced mode.
Open to suggestions on that one, just don't have the time to delve into this as I'm working on a very specific section of the new MPC Bible

It might be that some configs are just more suitable with legacy mode, or is this a bug?
Some stuff can be handled elsewhere, for example for a piano I would normally have different release times on the lower note keygroups but that can now be done via the mod matrix using 'key track' mapped to amp env release.
There's also the GLOBAL parameter setting for all the envelopes (bottom right of envelope), which the manual says controls whether the envelope is applied to the current 'voice' or entire keygroup. Not sure exactly what this means....