I imagine every MEMS producer will have a whole range of...

I imagine every MEMS producer will have a whole range of problems that emerge from the microscopic behaviour of the minute material components that are designed to move, especially when they contact or interact with other components. There is a whole subset of physics that takes on much larger roles when you get that small. Just for an idea of what that would be like imagine if you were assembling an appliance and all the components, screws, washers, nuts, and bits of dust, were made of either glass or rubber but were all highly magnetic, and they refused to go together or only went together in particular orientations. On top of that, they tend to stick together or shy away from each other inconveniently, or stay connected when they were supposed to separate, or bend when they should flex, or snap off if pushed too hard. These inter-surface forces are way more common at that scale. That is why MEMS is a hard technology to master.

No I was not aware USound was having problems. USound is quite a different device to AP. Their MEMS device components all vibrate in an analog manner I believe. It is literally a miniaturised speaker membrane. That could be a lot more problematic than an actuator that simply moves repeatedly, precisely and reproducibly between 2 positions. It is probably a lot easier to engineer that single motion to move consistently than one that moves in an analog manner at a whole range of frequencies. That is another of the reasons why I consider AP has having a far superior technology.