I think we have already heard reports of problems with some combos. It seems that while most mutations we breed are not life threatening (two copies of HG Woma and possible two copies of Spider look to be exceptions) there could be a cumulative negative effect of some mutation combos even though they aren't at the same locus so they can be combined. Hopefully most mutations are only skin deep regardless of what they are combined with but I wouldn't count on them all working that way.