E-6 is pretty much a go-no go as far as the actual processing is concerned.
Either the chemistry is right or it isn't. _Generally_ the more film going
thru the processor on a regular basis, the more likely the processor's
replenishment and temperature is going to be correct. If the machine runs 20
rolls an hour for eight hours a day, down time or screwed up processing is a
substantial financial lose. If the machine runs 10 rolls a day, bad
processing is not that big a deal. The biggest variation is the mounting of
the slides. Do they mount them straight? Are the mounts cardboard or
plastic? Are the slides clean or dusty? Can they cut the slides correctly?

Color print film printing (ra-4) can have significant varibles. Different
film manufacturers will have a different orange mask for their negs. It's
important that the printer be set up for the film's mask and for the paper
being used to make prints. Additionally there is the human element: does the
printer operator just hit the "N" button, or does he actually use some
judgement? If the negative is a bright light snow or beach scene, does he
hit the "N-1" key. If the negative is a bright red barn, does he hit the
"R+1" key. (The machine is going to try to make every picture meet a certain
average density and color mix. On my printer, a bright scene needs to be
printed lighter than what the machine wants. A negative with a predominent
color needs to have that color added because the machine is going to try to
subtract that color. Disclaimer: Processing machines vary.)