I worked on a system built on FAAS for a few months and it was (already when I arrived) one of the more well-tested systems I ever worked with, so it is clearly

I worked on a system built on FAAS for a few months and it was (already when I arrived) one of the more well-tested systems I ever worked with, so it is clearly doable. I took the system from dev to prod and much of it was boring in the very best sense of boring (also helped that I had a good team, although they were new to it as well). I had one major gripe with it (and everything Azure) afterwards: On dev there are absolutely no guarantees: components might go down half a workday and they don't care. And when you go to production the prices goes absolutely through the roof (IIRC $9000/year for the most basic message broker I could configure that wouldn't have the risk of being offline half a day, and every component is like this). So while it was really cool to work on a cloud native system if someone ever asks me to design one for them, that design will be presented with a price tag for what it will cost to take it to production.