Added to package.use:
media-libs/libffado python_single_target_python2_7 python_targets_python2_7 -python_targets_python3_4 -python_single_target_python3_4 -python_targets_python3_5 -python_single_target_python3_5 -python_targets_python3_6 -python_single_target_python3_6
This prevents libffado from requiring scons-3 and python-3, which causes libffado to fail to compile.
There is now a realtime group that you need to add your user to. I will try to look into having this done by the installer at some point, but do a gpasswd -a (username) realtime in the meantime.