As a long-time reef aquarist, I have a good bit of experience with PVC. My trial run of my Kia Soul build was done on a PVC base, because I didn't want to do all the work with wood until I was happy with the heights of things. My bed and my cabinetry were all on top of 1" schedule 40 PVC framing, but are now all wood.
PVC will indeed bow in longer stretches if there is weight on it. But it isn't the long stretches that break. What breaks are the joints where people didn't properly attach the connecting fittings. If you brace in short lengths, there won't be bowing.
The more bracing you use to eliminate bowing, the more joints you have with PVC fittings. The more joints you have, the more likely you are to have a break in the structure if you don't properly secure the fittings.
The bed platform I made was 66" long. I only had PVC glue to keep the fittings together, and there was no separation or breaking apart. If you are expecting more weight than an average person, or you don't trust PVC glue, then you can bolt the pieces together after properly gluing them (either regular metal, pvc or nylon bolts and nuts). The frame held up just fine, and now it's the base for a sewing table in my apartment (I extended the legs to make it taller).
The pvc fittings and bracing are going to add to the weight, and when you consider that all those fittings will made your cost much more than if you had used wood, you are probably better off using wood.
The only reasons I can think of for using PVC for building furnishings instead of wood, other than if you already have a bunch of pvc sitting around, are the lack of tools, the need to do a quick job, and the need for a material than can withstand water exposure without having to do any surface prep or painting.