Andrew's A arm broke because of a bad weld, it happens rarely.
As far as airbag and shock failure goes, yes they have been know to fail, we have had a few failures. It doesn't seem to follow any trend either over all design or brand.
In general they are reliable. They can have micro leaks but that's to be expected with a system with valves, air line, and fittings.
As far as spares go you have a very limited number of moving parts on a trailer, I'd carry a bearing replacement kit for sure, and possibly a replacement shock and airbag or an air-over depending on the design. Most people don't carry replacement bags or shocks.