@BuZz Reporting on success building Retropie on Lubuntu 16.04:
install from binaries does not exist, had to compile everything from source
some emulators are optimised for ARM architectures (or so packages say) - not sure how they're going to work over time
many emulators and ports failed to compile on first attempt; after 3 attempts, only a Doom port is still holding out (everything else reports OK, but haven't tested yet)
Mupen64plus was waaaaay too fast to be playable (tested on Mario Kart). Removed and re-installed from apt-get and everything seems to be OK (may be solution for other ARM-optimized packages)
Apart from that everything seems to be working fine :D thanks!!