I don't think it's too much to expect of the pi, honestly. I have it working 100% with my nes30pro's on firmware 1.69. At most I have to wait up to a full minute for them to connect, but so far they have worked without fail (once on 1.69 firmware).
I know there are multiple pieces at work here: hardware, software, and environmental factors. I was bringing this topic up in case it would expose issues other people might have, and potential solutions.
I'm aware of wired solutions, and their consistency. I have buffalo controllers, etc. The point of this was to see if I could get the sfc30's working, as advertised with my particular hardware/software setup, not to settle for "well it sometimes might work." If the answer is "it doesn't work" that's fine, but the recalbox documentation suggests the device is fully supported. I didn't see a caveat for the pi.
My experience so far with sfc30 and recalbox on pi has been subpar, and I would not recommend anyone get it unless they want to spend significant time dealing with technical issues. There are other wireless options that appear to work much more consistently.