I’ve got a few Feather boards and wings, I’m pretty happy with the form factor.
Generally with most of the ‘consumer’ embedded stuff it’s more about what framework you’re using rather than what the underlying platform is. So with Adafruit stuff that’s exclusively Arduino or CircuitPython, and both of those will try to abstract away a lot of the very low level code from you.
I wouldn’t be surprised if Adafruit have set it up such that you can switch code from one of their Atmel ARM M4s to this board without much change, and you can even see that in some of the libraries they supply (e.g. the NeoPixel library has a bunch of
#ifdefs for lots of different boards).
PlatformIO has a decent list of what boards support what frameworks.
Anyway, this is a roundabout way of saying if you’re specifically trying to get into the STM32 platform you might find this board a bit frustrating as you’ll need to do a bit of detective work to get non-Arduino framework code running on it.
On the flip side, depending on what the ‘great open-source STM32’ is you’re after, it might be easier to port that to the Arduino framework (or and then just go straight to using a Teensy, if that form factor works better for you).