I've checked a few out on 20" donor bikes, very light-not in a good way, usually painted steel, rim edge is rolled not formed.
The sidewalls aren't straight, cross section grows and shrinks.
Theyre marginal for 60lb kids, I don't know if you could keep them straight with an adult riding.
24 and 26, you'll probably get at least an aluminum rim rated for adult weight.
What size are you looking at?