As far as complicated goes, a BC from the shop's rental gear is complete. If it is weight integrated then the system is there. No choices about where to mount, what size pocket etc. No questions about wing size, tacoing etc. The jacket will float you at the surface. Also, theoretically, each student is wearing close to the same gear, so the first buddy checks are easier as the gear is 'somewhat' familiar from putting it on themselves. Now don't get me wrong, I'm a fan of BP/W, but IMHO, let's get them in the water and comfortable first. Once that happens then we can begin to coach and advise on the different types of gear. They will also have a baseline of experience to measure new styles/configurations against.
Just my .02!!
Why are these problems? I'm sure you have a good, general idea of what your average student looks like and how much weight they usually need. So you get yourself some rigs that are all exactly the same that will fit most of your students and then one a little bigger and one a little smaller. They're all wearing the same thing, except one of them may have a bigger plate than another and one may have some additional weight on them, not a huge deal. Even if I had weight integrated BCs for my classes, I wouldn't use it because its probably important for the students to learn with a baseline of a belt rather than the specific system that BC has.
The BP/W is actually a much simpler setup with far fewer bells, whistles, buckles, straps, toggles, thingamabobs, dohickies, and whatchamacallits for a student to ask "whats this do" as they yank on it and either destroy your gear or send themselves either plummeting to the depths or rocketing to the surface.