I tried to use FreeRTOS on two projects. The first was an Atmel Xmega. The second was an Atmel SamD21. FreeRTOS has its quirks, but it's pretty straightforward and easy to understand. Unlike Zephyr and many of the current slew of "IoT platforms", it doesn't try to be anything but a microkernel. Just drop the C files in your sources, tweak the master defines file, and you're good to go. Use whatever build system fits you best.

The one area (and this may be dated; my last attempt was nearly 2 years ago) where FreeRTOS let us down was in tickless mode. I think FreeRTOS's place of primary use was originally FreeScale and automotive, where running tickfull and powered all of the time makes sense. But we needed to be tickless and sleep lots. There was a hook for doing that, but it was definitely "on your own" and figure it what else you needed to tweak outside of the hook (like I say, this may have improved of late).

I got distracted by RIOT because they spoke at ELC and I got it in my head that it was more FOSS/open-sourcy than FreeRTOS. When I grew with frustrated with RIOT, I stumbled on https://www.osrtos.com and gave TNEO a spin and was really happy with it.

I've only played briefly with mbed, but had the same frustration as many of the others. It want's to be a full-stack end-to-end development solution. That demos well, until you find you want to tweak the build process for some reason. Or integrate it in your source control system some other way.