Any undercoating will chip off in flakes. Dried oil and road grime will smear or crumble off.
Solvent for road grime, heat gun for undercoating.
Undercoating is put on for sound deadening. IMO it accelerates all corrosive processes, so go ahead and scrape it off.
The most rotten cars I've seen were all undercoated. Generally, the Canadian cars that had the least rust were without. There are exceptions of course.