I'll chime in with some of my experiences, especially with a Gen 5 MOS. Trijicon now makes a $16 kit that solves the issues that plagued the RMR on a MOS slide initially. With the new screws I haven't been able to find any problems of screws backing out or sights going airborne after around 1000 rounds of +P 9x19 on a Gen 5 17 MOS, not an endurance test by any means but so far it is holding up as it should have in the beginning. However, the MOS system has the RMR riding much higher on the slide than a dedicated mill job. This isn't the end of the world but it is certainly nicer to have the optic lower like milled slides do.
I specifically bought a Gen 5 17 MOS because now I don't have to send it out for front serrations like the Gen 4 MOS and it will be ready to immediately accept an Aimpoint ACRO when it comes out along with any other future optics that are released. For serious use guns I will still always be getting a dedicated mill job because you are only losing flexibility and incurring some extra cost to get a more robust and in most cases lower mount with one less failure or movement point. That being said, I would still have confidence in using the MOS system if it was issued to me and I was unable to use a dedicated mill job slide.