I've found the undercoating varies drastically from brand to brand as well as just different levels within a single brand. And, NOT SURPRISINGLY, the level of quality seems to be directly related to the price of the undercoating.
I bought some of the 3M Proffesional Rubberized Coating, that states on the label it contains NO TAR. Boy that stuff was great, but at $8 a can, I couldn't afford to do the whole underside with it. The $3 a can stuff, sure enough, seemed to have a lot of tar and was very goopy.
The $8 a can 3M Pro stuff went on like spray paint, after drying it even looked like a thick coat of flat black spray paint, and it was tough, with a rubber feel to it, but it did NOT give when you pressed on it. The cheapo stuff, even after several days to dry, would rub away easily. The 3M Pro stuff within an hour could not rub away if you tried, you would need some serious tools. You could also tell by the coating, that it was clearly adhering to the metal.
I used the 3M stuff for covering the patches I welded in, and the rest was the cheapo stuff.