PhotoEngineer has commented multiple times that most commercially available BLIXes are inferior to separate bleach and fix steps. He should know, his name is on Kodak BLIX patents

Main issues according to his postings is that BLIXes are poor bleaches and poor fixers, which leaves some silver or silver halide on the film. This in turn can affect both D
min and long term stability. I have mainly used BLIXes so far (mostly because three bath kits are all I can get here), and I have seen yellow cast and higher D
min on my slides, usually not with the first batch, but when the BLIX has been reused a couple of times.