I am sometimes asked to use metal boxes on a Romex job, until they find out how much more it will cost.
I have yet to find a 1 gang plastic box that can handle the following scenario- three wire w/ground home run, 2 two wire cables & a receptacle. If there is a box big enough for this, my suppliers don't carry it.
Box fill is also a problem with multi-gang boxes since there isn't, in my experiance, 20 cubic inches per gang. Of course, maybe I've led a sheltered life.
Naturally, careful wiring layout can overcome the fill problems in some cases, but the multi-gang box is a problem for me since I do not run switch legs down from a light. Tack in a 3 way or two & the multi-gang boxes fill up fast.
Like you, I only buy one size single gang box, the biggest I can get. Cost difference is minimal & the work goes a lot faster.