Undercoating is supposed to be soft-ish and take hits from rocks and such that would, in theory, scratch the paint and start rust. It is hard to get off, but if it wasn't it'd be no good at all. I use it over POR-15 on bottom/unseen surfaces, if it's something that needs protection but needs to be hard and not attract dust and such I use truck bed liner. I've also noticed spray undercoating varies a lot in quality from brand/type to brand/type.
Now POR is damn hard when properly applied, it'd likely be alright on it's own from the hammer-tests I've done here and there, but for what it costs, better safe than sorry to me.

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One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K