Why does commercial E6 processing need 7 steps, and Cinestill only 3? There must be a compromise somewhere.
It appears that Anon Ymous has given some specific details and koraks has given more general explanations.
I'm a former large lab guy with pretty substantial first-hand experience in what we loosely call "process control," by which I mostly mean dealing with 1) the actual results on film and 2) the chemical makeup of all the processing solutions. I should point out that I HAVE NO DIRECT PROCESSING EXPERIENCE WITH E6 (although I have, on request, occasionally made troubleshooting opinions for company-owned minilabs - long-distant past). The chemical solutions are essentially very similar to certain other processes.
Anon Ymous referred to a combining of bleach with fix in the Cinestill system, something that is also sometimes done in aftermarket C-41 (color negative) systems. I can elaborate on this, which may help in understanding some differences between commercial vs hobbyist processing. It's gonna be some tedious reading. Probably only a handful (or two) of people on this site will even give a hoot. But that's who I'm primarily writing this for, I guess.
In the commercial processing world it's generally important to hold chemical costs down as well as to deal with effluent issues (which can be seen as another chemical-related cost). So the number one cost-effective trick is to "replenish" the processing chemicals. This greatly reduces the amount and cost of the chemicals.
Essentially, a replenisher restores a processing solution back to its original aim specification (and performance). In a developer, for example, one doesn't have to just discard it after some limited use. As film is processed two main things happen: first is that development byproducts build up, and second is that some of the actual developing agent is "used up." So a proper replenisher would both dilute the byproducts and restore the developing-agent concentration back to aim specs. As this is done the developer stays in a steady-state condition, always good, never going into a marginal condition. In a C-41 developer replenishment makes it possible to reduce chemical costs by a factor of about ten times. The downside is that it takes some expertise (and additional labor costs) to keep things finely-tuned. For a large lab the extra labor costs are a small proportion of the chemical savings. For a small-scale hobbyist the balance goes the other way. So hobbyist oriented systems tend to make it simpler for the user. Use it as long as it seems more or less ok, then get rid of it.
But I digress... let me get back to the bleach and fix (again being more specific to C-41 color neg). The bleach has two fundamental components - the bleaching part and the halogen supply part (typically KBr, potassium bromide). As the bleach works on film the bleaching part becomes "exhausted" by being chemically "reduced." But... it can typically be restored by simply exposing it to oxygen (bubble air through it in commercial processing, perhaps shake it up in a larger bottle for hobbyists). But the bromide part will eventually be used up. Additionally the bleach is gradually diluted by "carryover" from whatever is in front of it. So... the proper bleach replenisher has to overcome the dilution effect plus add some extra bromide. Additionally the "exhausted" bleach part must be made "active" again (by simply aerating for certain types of bleach). This replenishment method makes the bleach very cheap chemically AND drastically reduces the waste volume. But... someone has to know what they're doing to keep things in control.
Now for the fixer... conventional rapid fixers go "bad" in two ways - first, they have limited capacity to pick up silver. So this helps to set a limit on ability to keep using. Second, the active fixing ingredient needs a "preservative" to protect it from oxidation (if the fixer gets oxidized too much it will go bad). It seems that a commercial user would ideally load up the fixer with as much silver as possible. But... there is a big problem with this... as some of this fixer eventually gets carried over into wash water it brings along enough silver (concentration-wise) to start a 4-alarm fire in the minds of environmental regulators.
In the US, at least, commercial processors generally cannot be connected to a "sewer" (a POTW, publicly owned treatment works) without a "sewering permit" which specifies a number of conditions which must be met. One of these sets a silver concentration limit which might possibly be low enough to be essentially unattainable by most people. In my experience regulating to something like two-tenths of a milligram silver per liter is not uncommon. (1 mg/L is roughly equivalent to one part per million, so two-tenths mg/L is equivalent to one part in five million. The numbers are roughly akin to controlling the population of New York City, pop around 8 to 9 million, to within one or two people.)
Back to the commercial processor's use of fixer... they would ideally use a multi-stage counter-current flow fixing setup that has highest silver concentration in the first fix tank, with the final fix tank containing low enough silver that it doesn't put the wash water over the permit limit. This is kinda a fine balancing act with a processing lab capable of doing silver analysis below 1/10 mg/L (silver-estimating paper can maybe see down to 1/2 g/L = 500 mg/L; roughly 5,000 times higher than desired analytical capability). Fwiw the way the photo industry dealt with the silver effluent limits in minilabs, back in the 1980s as I recall, was to invent the so-called "washless" systems. So a washless minilab did not need a sewer hookup. The total amount of effluent was greatly reduced, such that it was feasible to have (and pay for) a Hazardous Waste Hauler take it all.
So back to a lab's use of separate bleach and fix, vs combined into blix... when they are combined the lab doesn't have the ability to optimize the replenishment of either bleach or fix. And if they try to keep the bleach portion active by aerating, this destroys the preservative for the fixer portion (typically the preservative is sulfite ion, being oxidized to sulfate).
Now, if you're a hobbyist, you might be running something like 5 or 10, maybe 20 rolls of color film in a batch. Maybe more? For smaller amounts I'm guessing that a film blix MIGHT be ok for a while (total guess). Maybe the modern chemical outfits have got things tuned well enough to work ok? If you're running enough that replenishment might be worthwhile then you're likely better off having separate bleach and fixer. Then you'll have the ability to aerate the bleach to keep it healthy over an extended time (virtually forever if you replenish it). Plus you could run a two-stage fixer with optimized replenishment rates. On the other hand if the hobbyist doesn't care that much about recovering the silver, well, whatever. I doubt they'll have any regulators stopping by to check their effluent.
Anyway, once one sees the possible complexity of running some given process, one can better appreciate why the simplified aftermarket systems exist. Mix it up according to instructions, use it as far as you feel comfortable going then discard. As long as you're gonna scan the film you can probably deal with about anything, process-wise.
