Personal experience.
It leaves green crappy crusty fuzz on all metalic surfaces it comes in contact with...or I should say the combustion products given off by the burning of methane do so. I don’t know what that green junk is, but it eats holes in non stainless steels real fast.
Aha, yes, if you’re talking about raw “swamp gas” collected au naturel and not commecially purified, no question about it, you’ll have some sulfur gases in there. Those will stink up the joint and make sulfuric/sulfurous acid which will corrode the hell out of many metal parts. Could get some nitrogen oxides in there too, probably less, but still makes nitric/nitrous acids on combustion. Yeah. The gas itself would run an engine OK, but the pollutants would eat the insides.
Normally, commercial nat gas would be purified, probably by liquifying it, and those pollutants would be separated pretty easily. So you’d have pretty clean methane with a little ethane & propane as “pollutants”.