That is true! Developing the slide film is similar to developing the negative film. It's a strict procedure and requires no operator judgment. Most labs if they do it right the negative or slide should come out fine. However, in the printing or scanning stage of the negative, operator judgment comes in and because the lab tech is not you and so often the result is not to your liking.
If you print (in the darkroom using RA-4 process) or scan your own negative then there is no advantage in this respect by using slide film.
Standardization isn't the issue, the C-41 process that the OP can't get done well is just as standard.
E6 has all the same problems of quality and operator mismanagement
These processes are also still capable of adjustment, like push/pull.
Mark Barendt, Beaverton, OR
"We do not see things the way they are. We see things the way we are." Ana´s Nin