?'s on mixing some paint

i wanna know what the flaws of mixing a univeral base coat with a acrylic enamel?? i mixed this two together and they turned out ok. then after i sprayed it on a test fender and so far it looks good. when i was spraying another truck with clear coat i decided to spray my test fender with it and seems ok. but ive always heard they want mix blah blah, so i went to the local paint dealer and they said u cant mix those two becoz they want, and i told her all that ^^ and she looked at me like i was stupid, so my question is whats the flaws of doing this?? thanks