The black parts of the keys indeed don’t do anything different than the rest of the key, but you can play them differently.
First of all, they’re all the same height and length, so if you want to ignore conventional keyboard format and give equal weight to all notes, you can play up there. You can also gliss from one place to another, going through ALL notes at the same physical height. I like playing some of the percussion patches (Balafun is a good one) up there. I also find they’re less sensitive, since the keys pivot from the rear you end up playing very close to the pivot point. That can be a plus or a minus depending on the sound.
I haven’t played any other MPE surfaces for any significant time, so I can’t compare directly. What I can say is that for me, the Y control system on Osmose makes sense, and Polybrute is doing a similar thing under a different name. You have very fine control of the sound with any amount of pressure on the key. It takes time to get used to that. If you sit down to play this and you just push keys until you feel the bottom of the key for all the notes you’re pressing, then you are not going to get different Y values for each note. It is underwhelming when you play that way, and doesn’t really sound any different from a normal synth. You can still push into aftertouch for some notes at different amounts relatively easily (and it basically doubles the expressivity of the sound), but training your fingers to not just go straight to that switch point takes some time and is different to any keyboard you will have played. Even a seaboard is registering the same thing, but mapped in a different way.
For pitch bends, you have a few options since you are right that there is no way to continuosly bend pitch polyphonicaly. First, on a per patch basis, you can determine how far you want the X axis motion of the keys to be able to bend pitch - anywhere from a few cents to 96 semi-tones. You can also decide how sensitive you want the keys to be to initial position. At min, they will always start in key and then your motion will add or subtract from that. At max, they are sensitive enough that you will pretty much never end up in scale. It’s been a while, but I think you can also edit all the response curves for X, Y, and Z dimensions as well as normal velocity/AT (meaning you can send one value to the internal synth and another to an external synth if it’s just in Poly AT mode). But, as you lift all the keys will return to their original pitch so you are correct that this cannot function as a continuous slide from one chord to the next.
Personally, that hasn’t bothered me, but that is not something I’ve had access to before either.
The other setting you have access to on a per-patch basis is pressure glide. This lets you move a single pitch between two keys by varying the pressure between them. You can set the distance between which any two notes played will sound a single pitch and then by varying the pressure between those two notes you move the pitch freely. Not polyphonic by it’s nature (unless you can play 4 or 6 notes and precisely vary the pressure between pairs). But it is pretty flexible, in theory it is possible to play multiple notes at once and bend them independently. You can set the maximum distance between notes from one to 96 semi tones. At one semi tone it’s essentially a fine pitch. At 96 you’re playing more like a theremin or a mondes. But you can set any range in between. It will always limit your chord choices if you are trying to play polyphonically.
