This really is good advice...
screw or nail the box to a stud...
Sheetrock screws are your friend here. Cut a small piece of 2by4 and slip in in the hole before you install the box and hold the wood in place inside the wall right next to where you are installing the box with one hand and screw it directly thru the drywall with the other and put 2-4 screws in it. Repeat on the other side. This will give you extra support so that when you install the box, you can run a couple of sheetrock screws through the in-side of the box and into the 2by4; plus, this will also give you something inside the wall to screw your fixture into if that becomes necessary.
Most of the larger bathroom fixtures I ever installed also came with butterfly anchor bolts to distribute the weight of the fixture across drywall. If you expect children to hand things from them or swing like monkeys on the fixture, I would DEF advise adding bracing behind the sheetrock or screwing the mounting brace with wall anchors...
Most days tho, in most bathrooms, on a VERTICALLY MOUNTED fixture...a pop-in all by its lonesome is plenty. HORIZONTALLY MOUNTED is a whole other animal.
Just remember that you dont hang Christmas lights on it and you dont use warm glowing bulbs to dry wet socks; advise children that games of "Tarzan" need to be kept outside...and a pop-in will do just fine....per my Humble opinion, and as per NEC CODE, which says...
ARTICLE 314
Outlet, Device, Pull, and Junction Boxes; Conduit Bodies; Fittings; and Handhole Enclosures
*314.20 In Wall or Ceiling. In walls or ceilings with a surface of concrete, tile, gypsum, plaster, or other noncombustible material, boxes employing a flush-type cover or faceplate shall be installed so that the front edge of the box, plaster ring, extension ring, or listed extender will not be set back of the finished surface more than 6 mm (1⁄4 in.). In walls and ceilings constructed of wood or other combustible surface material, boxes, plaster rings, extension rings, or listed extenders shall be flush with the finished surface or project therefrom.*
*406.4 Receptacle Mounting. Receptacles shall be mounted in boxes or assemblies designed for the purpose, and such boxes or assemblies shall be securely fastened in place unless otherwise permitted elsewhere in this Code.*
*(A) Boxes That Are Set Back. Receptacles mounted in boxes that are set back from the finished surface as permitted in 314.20 shall be installed such that the mounting yoke or strap of the receptacle is held rigidly at the finished surface.*
Master Electrician, Automated Systems Engineer, 25+ years exp.