I’m currently working on a product using rust on stm32 hardware. I’m very thankful to be using rust instead of c/c++ for a variety of reasons. That said, so far the big pain points so far have been:

Each HAL implements different things, or the same thing in different ways. I’m in the middle of switching from the stm32f1 i started on to the more capable stm32f4 and it’s been a painful switch. I assume this would also suck in other languages, but it seems fixable in rust. There’s also widely varied support for other MCUs

Lack of emulation. You can emulate an arm cpu in qemu, but there’s no tools for mocking out hardware - which means testing has to happen on device (or you need to fragment your application into qemu-testable pieces)

Shared memory / global initialization is overly complex. If you have a resource (say a gpio pin) that you want to set up in main but use in an interrupt and nowhere else, you have to use like five layers of abstraction to “safely” do that.