I've had an ARB on the front of my 1988 XJ Cherokee since around 1996, and a Kaymar rear bar since around 1999. Probably over 150,000 miles on them -- around 300,000 one tends to lose track.
I've been satisfied with how both have held up. The ARB is a bit faded, as noted earlier. The powder coat was broken where it was mounted, and there has been some corrosion (which I've stayed on top of with POR-15). At one point a tiny bit of corrosion started on the antenna tab on the top tube, and ARB sent me a touch up kit of matching paint, which worked well.
The Kaymar has no corrosion, but I used POR-15 pre-installation on it's backside and it seems to have a zinc-based primer under the black coating (which I'm not sure is powder coat).
ARB used to offer (in Australia) commercial bullbars that were galvanized, not powder coated, if my memory isn't wrong. I think my old home-market ARB catalogs may show these.
I think with careful installation and a little bit of care, you'll get a LOT of years out of one of these.
Don