Strangely I was mainly processing faster E6 films, initially 400 ISO then when introduced the specialist P800/1600 films from Kodak & Fuji. These were optimised for push processing and I found it better to process my own than send them to a lab in the nearest city.
I started using the Photocolor Chrome 6 kits very soon after their release and they had been vigorously tested. There were concerns not long after because the Blix's in some competitors kits being imported from the US were found to be inefficient in magazine tests, it didn't need a chemist to spot this either.
I know that Ilford had tested the Photocolor C41 & E6 products and had found no problems, at the time they were selling re-badged colour films under the Ilford brand name, I still have an unprocessed roll of Ilfochrome (E6). Silver retention is one key issue they'd have tested for.
While I agree with Ron that a Blix may not be theoretically ideal, some better designed Blix's are quite different and work extremely well. I have seen the difference between poorly blixed film C41 & E6 and a well blixed or/bleached then fixed film and can spot the difference. I've also seen comparative tests done using a Chrome 6 kit & a Commercial (high quality) E6 lab and no-one could spot the differences.
In answer to Jack's question regarding fading, I've only seen it with slides that have been very extensively projected. I've not noticed any fading in E4 or E6 films both commercially and self processed and many are over 30 years old. I started processing E4 films around 1972 (E3 type kits).
However without a reference to compare to slight fading might go unnoticed. Harrods store in London had backlit photographs (Duratrans or similar) alongside the lifts on each floor, even after a day if they changed one image in a set they had to change them all as the fading was already just noticeable. Leave a slide in a projector for 6 hours and it will be quite significantly faded.