Like with many things, it seems easy till you start to work and dive in. With materials it comes down to availability, usability and price.
I have not found ideal solutions for 4x4 underbody coating.
Not sure what it is used today from Graz, looks still same but on older vehicles the thick rubber like coating is PVC top of EDC. Due health and environment reasons today in auto industry PVC is generally much less used. Under the car it is very durable and adheres very well even to smooth surfaces. Heat will make it brittle and if moisture gets through it will be a good nest for rust. There are very few PVC based products available, I found something in old Dinitrol catalogue (20 litre cans), and if still in production it is special order and working with it is health risk.
Available are wax and bitumen based products what will remain soft and sealing. Applied over properly pained surface. It is also OEM solution for newer cars. But it does not offer good protection against stones and off-road abuse. New cars get away with stones because all underbody is covered with plastic liners. Most of those products promise ‘dry to touch’ but I have seen some bitumen ones where working under car gets very messy.
Then there are underbody and stone protection products what are thick soft paint-like coatings. They will chip if damaged and some are actually porous (that gives them ability to take hits from gravel stones, dampen sound and save weight) and will entrap moisture creating yet again good place to metal rot. Many of those products are 1K. All this is maybe good for street cars.
Bed liners are most durable. Of course the line between underbody protection products and bed liners is bit blurred and by its properties the PVC is suitable for bed liner. They usually form strong, thick, solid yet flexible uniform surface. Again there are different products for different prices. And as with underbody coatings bed liners are as good as the prepared surface underneath. People sometimes fail to recognise that being most durable and lasting coating the weakest link will become metal underneath, with only paint or cheap stone chip there would be nothing for that time. Yes, there is an issue that smooth intact surface makes illusion that everything is okay.
Dealing with that headache I went and got whole shell underside covered with Line-X. Literally bombproof 2K material. Bottom got sandblasted again, right away went to facility where Line-X technicians applied proper primers, seam sealer and next day the coating. Work and material was not greatly more expensive than some other products. But shell has to be on rotisserie. I have no illusions that there will be no spots where moisture finds its way underneath the coating and rust will starts. Like one book explained: oxidation is a thermodynamic process, rust is more stable element in nature and inevitably all steel will become rust in some point.
Issues with painting are even more complex. But totally agree that there are almost no places for 1K product use.