A couple of things comes to mind in your case. Strictly speaking about coilovers, if you drive in snow, and road salt is used, you can count on rust/corrosion. The main issue is with the height adjusters which tend to seize. This makes seasonal adjustments a difficult but attainable process. Like anything threaded, corrosion is not your friend.
The industry addresses corrosion in a few different ways. First off, most coilover bodies are aluminum or galvanized steel. Manufacturers use different treatments like anodizing and zinc plating to ward off rust. This works for a while, the length of which is determined by how much exposure to corrosive elements there is.
A more expensive option is KW Varient coilovers which feature off the shelf industry leading INOX stainless steel bodies. Check out the pic below:
The second thing is length of warranty and where the work is performed. Warranty is not an indicator or how well a coilover performs, so a 1 year warranty item may perform as well as an unlimited warranty. Having an extended warranty is good to have. Where the rebuilds are performed (in the USA or outside of) comes into play when shipping your coilovers. First is expense to ship. Second is length of time to receive your parts back.