They don't usually have the UV protection and are thinner. I use them for sealing coats sometimes and regularly for spars for an annual maintenance coat. Better more expensive varnishes seem to build faster(less coats) so you get longer protection for less application coats.

I use a lot of Rustoleum paints(both urethane and regular enamels) with good success. For cheap paint, it's pretty good stuff.

I don't use the urethane varnish much if at all for exterior wood but will use it below. I like the easy on, easy off of regular spar varnish. If you try the Rustoleum, let us know.