> I mean some of these things have bugs still, but generally when you have something of fixed scope then there are less bugs over time. Especially if you're restricting your scope to something simple

I'm guessing that smart contracts tend to be relatively short and simple? I mean I would never bet on an entire web/mobile app being bug free because the surface to attack is enormous but e.g. a sorting algorithm or something small in scope can be formally verified to be bug free as long as your specification is accurate.