I would have nothing to do with FIUV. They are not Traditional, and for verification you can check Rorate Caeli, wherein a recent post of Una Voce has explicitly "accepted" new Prefaces on behalf of the laity. They are covering up this fact with a series of "position papers" railing against such things as alter-girls (sic intentional), Communion-a-mano, and so on. These papers are nothing but a smoke screen, and it seems imminent that the Motu-Masses are soon to be corrupted. It may work out in the end, insofar as FSSP and similar will, at least in the short term, become a feeder system into truly Traditional fraternities which don't answer to a horde of (*).
So in summary, Foederatio Internationalis Una Voce is selling out. It's not the same group which Davies headed, and it's current leadership are arrogantly pursuing their agenda, despite having been warned repeatedly what evil fruits their current tack will bear.