I was wondering if there's any film for which the monobath produces truly remarkable results that wouldn't be easily possible to get with regular developers.
I think it fairly unlikely. The product designation Df96 makes it seem Cinestill intended their monobath to simulate D-96 (even though they used phenidone rather than metol), and if they hit their mark, then D-96 and a separate rapid fixer ought to generally produce as good or better results compared to Df96 -- if only because you have independent control of time/temp and agitation, and don't have to try to control a race between development and fixing. Even if the developer component isn't just a phenidone-substituted version of D-96, there is such a bewildering array of developer formulae that calling any one "best" for some one particular film is just a way to start an argument.
What Df96 does better than any other product I know of is put all the chemicals you need to process B&W film (aside from a wetting agent for the end of the wash) into a single bottle, let you pour them all at once from a single graduate, and offset loss of activity as it exhausts by exhausting the fixer and developer at about the same rate. There are a couple other monobaths on the market -- Famous Format (formerly New55) sells a couple, one that's an evolutionary follow-on to one I threw together back in 2003-2004. I haven't seen any information about controlling contrast (i.e. pushing and pulling) with theirs, nor the extensive list of tested films and expected speeds in "normal" process that Cinestill provides. As well, I haven't seen any option to get the Famous Format monobaths in dry form -- which saves significantly (both purchase price and shipping cost) over the premixed, as well as arriving sooner if you buy from them because it doesn't have to be shipped ground.
On the other hand, with its limited use (roughly sixteen rolls if you shoot enough, less if it takes you more than a couple months to use it that many times), Df96 is anything but a cheap way to process film. Yes, much cheaper than sending it out, but nearly any one-shot or replenished dedicated developer and reusable fixer will cost less per roll to use. To pick an example I've just started, Xtol use 1+2 costs something like 30 cents a roll and basic rapid fixer is less than that (if you mix your own fixer from pool chlorine reducer, it's
really cheap) -- but if you replenish your Xtol, you cut that cost almost in half (more than half as soon as you start your second 5L pack and don't have to dedicate a liter or two for working solution). Home mixed developers can be even less than that; even without access to Costco discount/bulk pricing for acetaminophen (paracetamol), and having to buy lye from Amazon because local stores don't seem to carry it any more, I can mix Parodinal (exact Rodinal work-alike) and get costs of about half the best replenished Xtol can do. Replenished Mytol likely runs close to Parodinal, if not a little lower (depends what you pay for phenidone and sodium ascorbate).
What it all boils down to is that Df96 is
best at really only one thing: getting people into (or
back into) processing film with a minimum investment in infrastructure. Changing bag, small or medium Paterson tank, jug or bag of Df96, bottle of PhotoFlo or equivalent, a package of negative pages and some clips to hang the film, plus a shoestring to hang it from. Everything else can be added on later if you like it, or when you have more space and time. It's convenient for travel processing, in some cases, but the Ilford/Tetenal sachets might well do a better job, are cheaper for just a few rolls, and take up less space in luggage.