Would you say it's easier to spec out hardware than software? I don't work with hardware, but from a layman's perspective it seems like you can (and have to be) utterly exacting when it comes to product matching designs, with said designs themselves being very exacting, i.e. using physics equations.

With complex software, it's pretty dang hard to layout an entire program in specifications without going down the provably correct approach, which likely will run into limitations when the real world is involved. Waterfall dev for operating systems of the past come to mind.... Probably the best you can do is be very stringent with your coding standards, something space agencies are the masters of.