I’ve seen plenty of failures but never due to mixing brands of epoxy and urethane paint. I’m sure the manufacturers and jobbers like to use that excuse because it absolves them of all liability….

Quality epoxies and urethanes have no compatibility issues. Quality being the operative term…

not an excuse, it is real.
The main thing for others is there is no reason to take the chance. It is real easy to stay in the same system throughout and eliminate one other avenue for things to go wrong. Zero advantage to mix. People try to be a chemist or save a buck. A proven system that has been designed, developed, tested and beta tested is always the best option.

Epoxy is inert by its very nature. You and I will have to agree to disagree. I’ve been doing what you advise against for 20+ years with never an issue.

This multiple show winner has epoxy, primer, base and clear from four different manufacturers.