until i create batocera at the beginning of 2017, i was the main contributor of recalbox.
on mars 2017, batocera = recalbox 4.1 + fixes + new features, among them, the more robust and fast upgrade system (you can upgrade and downgrade easyly), saturn, jaguar, up to date linux kernel, better x86 support, new wii features, ...
dosbox already works then batocera.
3do : it is supported, however, it's something i never test.
c2 saturn : not sure
c2 dreamcast : probably never.
c2 has still some issues on batocera i've to work on :
- bluetooth controllers
- n64 low performances
- ps1 low performances
- kodi hd low performances
so in 2 words, it's not good for you.
i like the c2, but it still suffer of some issues.
i were working last days on the c2, for the moment unsucessfully to look for optimisation (because there are possible, on lakka for example, i was told it works better)
best platform is probably mini x86_64 computers on which you can run everything, or xu4 which is more powerfull.
rpi3 remains a good platform.