The different kinds of developer machines
There are two basic kinds of film developing machines.
The first type is called a "roller" or "cine" transport. Mini lab machines tend to be this kind. The machine designers try to make them as dust-proof and make them touch the film as little as possible, but if you get dust inside of the machine in the right place, it will leave a nice scratch in your film.
These machines are designed for a single set of timings, so you can't push or pull the film. If the motors inside aren't properly set up, you could get uneven developing.
There are some fairly huge machines like this, designed to develop thousands of rolls at once and those are likely maintained better.
These processors can almost always handle 35mm film and sometimes can handle 120 roll film. However, a special adapter is usually required to develop 120 film and most places don't bother getting it. Because they are derived from the machines designed to develop thousand foot rolls of movie camera film, they don't sweat over long rolls of film.
The other type is a "dip and dunk" machine. With these machines, film is put on a hanger with a weight on the bottom and it is dipped into successive deep tanks. These machines will take any film that can physically fit in the tank, however they can't handle a long roll (by a long roll I mean a roll longer than a standard 36-exposure roll of film). These machines can easily push or pull film by changing the rate of transport between the tanks. They don't need to touch your film, so if there's any dust, it won't cause a long scratch mark.
In both cases, the machines can ruin film, of course. If the chemistry has been allowed to go whacky or the machine dies while developing or things like that, you can get ruined film. But only high-end places tend to have dip-and-dunk machines, so they tend to be better maintained. And low-end places that can only afford to hire people that don't care too much about your pictures will have roller transport machines and won't maintain them particularly well.
