I don't think there is much wrong with a blix for personal use, and i doubt it would affect image stability, if anything traces of silver would increase it because bacteria or mould could not grow, this is a strength of B/W film! I have tried separate bleach and fix and blix for E-6, no visual difference, even projected. Yes special equipment could probably pick it up (i cannot spell the name of what is used), but my work is visually pleasing to me and others who see them... hence i use a blix, it is easier to manage one solution rather than two. A blix will not keep as long however. I got two months out of one in an airtight bottle. Stay away from monoconcerntrate ones, you are looking for trouble! Bleach will keep almost indefinitely! I use mine to exhaustion though, if you are not using the chemicals much, go with a separate bleach and fix for the sake of keeping properties.
If there is any other reason why a blix should not be used do mention it though!