Most European legacy airlines have taken cadets and low houred DEPs into the RHS of narrow - bodied jets for years. Although, the training departments are well geared up for this level of experience. Longhaul/ wide-bodied flying to the more remote parts of the World will be a challenge for any Junior F/O, particularly if the unthinkable happens and there is an incapacitation in the LHS.
The risks are heightened if the stars align and the whole crew is inexperienced - crew pairing restrictions or not.