So I've contributed a couple of (tiny!) patches to Zephyr and wrote my own board support / devicetree setup for a custom Cortex-M4F setup that I designed.

Zephyr is awesome and the community is awesome, IMHO.

Using CLion to manage the CMake-based build is awesome.

What's not quite so awesome? The plethora of Kconfig items. Even with "make menuconfig" it can be very difficult to keep track of what you've configured in and what you've configured out.

For 90% of most projects, I'd expect the defaults to be sane. In fact, probably better than the ones an inexperienced dev would select.

But when you want that last 10%, it can be a real challenge.

I've had much more luck with Zephyr than I did with FreeRTOS or mbed (and I've used mbed a lot...)!