SAFETY act can be used to certify a product, or a process. So you can develop a system that is not based on specfic brands of products.
As I recall, SAFETY act caps liability at $10m under certain circumstances (consult your lawyer, not me) so I would certainly expect lower liability premiums since you'll need less coverage.
For quite a few years, there were specs requirements for SAFETY act, since it also limits the owner's liability. Most DOD contractors and many security integrators I know obtained certification/registration for some part of a security system, and as you note many manufacturer/vendors do as well. Since the process isn't that difficult- many companies I knew used an experienced lawyer, but others did it themselves, I would consider it a no-brainer from a business point of view.