I was working with monobaths for a while and occasionally come back to it as circumstances demand it. The attraction at the time was to be able to process film in limited space with fewer trays, storage bottles, etc. Another attraction might be to process sheet film with less handling, say one sheet of ULF at a time.

In fact the results were quite good, and if you like the Type 55 look, a slow to medium speed traditional film (I was fond of Efke PL100) in a monobath (I was using mainly fx6a) is kind of it. Haist argues that you can tweak a monobath to give results similar to D-76, and that fits with my experience.

The problem with a monobath is that without exotic and expensive chemicals, it tends to self-destruct once you start using it, so you have to mix it up and use it in one session, or at most within two days, or fixed out silver, the fixer, and the developer all start interacting to produce sludge and to exhaust the developer. You also need kind of a lot of developing agent to compete with the fixer, so it could get costly in large quantities.

There's a long monobath thread around here with lots of formulas, info, and sample images.