The whole truck bed liner paint concept is not very old.
It's only been around a few years. The Rhino Liner is likely the most recognized name in liners that are professionally installed. There are a lot of other systems however and in many cases local installers are using equipment and materials that are from a lesser known outfit like the Vortex bed liner or maybe Speedliner.
All these bed liner paint installers offer what seem to be unique features. None are exactly comparable. You may not realize it but neither are the diy truck bed liner products the same.
If you read on the do it yourself bed liner sites you will even get quite different ideas on how to apply roll on bed liner coatings.
The differences are mostly about what has to be done to get the truck ready to paint. Some of the newer coatings make bold claims about application over rust.
Some make the installation appear to be as simple as just a hose down and roll it on.
Keep looking though and you get these stories about how this paint is not any good as it just peeled off and so forth. Excuse me, but if it peeled off the fault was likely not with the material but with the installation.
Getting a truck bed paint on liner installed and well stuck is mostly a matter of getting the surface to be coated clean and properly etched.
What it takes to get that done is not complicated but it really isn't that easy. That's especially true if what you have is a beat up older truck. If that's what you have the preparation is even more critical.
Get truck bed liner paint that is made with multiple parts and you more likely will get a paint that is more like what you would get if you chose a liner coating from one of the pro liner installers.
The Rhino Liner or Vortex or Line-X are actually quite sophisticated coatings and tough to beat for protection and good looks.