Zephyr is quite nice and I've personally contributed some stuff, the general issues with it I think stem from IoT as a whole. People build products and move on to the next one. So sometimes drivers can be broken at times. It's still a RTOS and leads to common RTOS issues. The most frustrating to me is stack sizing. It's very easy to undersize a stack for a thread and not find out for quite awhile until something goes annoyingly wrong.