Hi Bruno,
The operation is quite different in that the group/track/layer buttons are now accessed through the Selection row. The selection row function is in turn switched by the 8 buttons around the larger wheel knob. So now you would choose Track; this gives access to the tracks on all 16 buttons. The Group structure (G1/2/3/4) is preserved to maintain compatibility. A big benefit is that multiple selections are now possible, which would have been quite difficult with the old workflow. Another benefit is that the Selection row can stay active while using the GP buttons above. Layers are accessed in a similar way. Hopefully the labeling is more intuitive now, and users don't feel limited to 3 layers of trig/para. Bookmarks are now brought out to the front panel.
With the MEC buttons, there's functionality not explicitly present in the v4 (e.g. Live). Also, each encoder has an independent switch, so no one should run out of buttons. Normally the switches would be assigned to acceleration though.
Maybe Peter can fill in anything I've missed.
Best,
Andy