IMHO, it depends on the application. If using flush speakers (ELK) I use boxes, MPLS's or LV rings. For KP's , I usually prefer to mount to sheetrock, since most of the times, it's easier to center the unit above the switches rather than trying to monkey around with a box that may or may not line up (or scab in wood). Also, retro boxes and rings tend to stick out from the wall, not allowing the units to sit flush with the walls.
I can't say I do the same with system boards, but the Elk enclosures do mount to them nicely.
PIR's are a "never" and GBD's depend on the unit, but I usually use G-techs, which are a lot smaller than a single gang plate. Smoke detectors in new construction are an always. I tend to use boxes only in "exposed work" where I'm dealing with studs and joists and no mounting areas, but retrofits are a different story. A lot of time, especially when devices aren't chosen prior during the rough, a gang box makes it more difficult since the footprint is already chosen, so if a smaller unit is chosen or the dimensions are different than the gang box, you're stuck.
Conduit and systems are a different game, such as installing KAM's (Elk) and readers.