It's true that if you want a really tough coating over the top of a pickup box that the drop in truck bed liners are tough to beat. However there are some plastic bed liner problems that are almost legendary in nature.
The plastic bed liners can wreck a truck bed. It doesn't have to be that way but it does happen and here's how.
The hard plastic drop-in bed liners are all made to custom fit a particular model of truck. Of course they aren't molded in place so there is a little bit of clearance between liner and bed. It has to slip in of course. What often happens is that little bit of clearance is enough that the liner can move around a little if it isn't really a tight fit. That can happen just from wind passing over the liner for example.
That little bit of movement is just enough to wear away at the paint on the bed and under the liner.
That of course takes place out of sight and out of mind.
That's bad enough but there's more.
Moisture also gets trapped under plastic liners. That results from rain that gets under the liner or just condensation under the liner. As long as that water can get to the drains it's not such a big deal. Let a drain get plugged or let the water miss the drain and you get this perfect damp environment in the dark area under that plastic. It just is the perfect spot for all sorts of corrosion to get started.
Mix that dampness with bare metal from worn away truck bed paint and you get the start of rust and corrosion.
Only one liner of the drop in bed liners avoids that trouble as far as I know.
The Dual Liner truck bed liner uses rubber on the floor and not plastic and thus gets around the wear of the floor problem. It's different in some other ways as well, all in an attempt I'm sire to avoid most of the plastic bed liner problems.