There are several grades of rebar from the stuff you get from the local builders' merchant to what they use for high rise buildings etc.
I believe at the bottom end the spec is defined more or less by the minimum tensile strength, easily and cheaply achieved by making it from general scrap and not being too fussy, so the composition is variable and it could be anything from ropey mild steel to hardenable steel. Don't forget they are selling rebar for a specific purpose and to be fit for that purpose, not as mild steel bar to a mild steel spec.
As for projects, it has to depend what they are, bearing in mind that the composition of the steel is variable. Hoops to support tomato plants, trellises, pokers etc, fine. Anything where there's a price to pay if it fails and I'd say not.