I don't think that's true actually. Not only do you get fewer parts in total, you also very quickly get to a scale where tracking part wear in detail becomes sensible. Slightly bigger scale and you get essentially 24/7 engineering coverage.

Yes a failure will affect a larger group of people, but it's also much less likely to occur, and much more likely to be fixed quickly.

As a thought experiment imagine everyone running their own privately owned diesel generator. Would that be more reliable than what we have now?

I see where you are coming from, because it’s usually the case that centralizing has economies of scale. The surface area of moving cold air and the damage a missed cold spot causes from condensation or a too-cold spot causes an ice jam seems to be a big materials challenge.

Cities have centralized steam systems for nearly 100 years, so there are probably different sets of challenges with central AC or folks would have figured it out? It might be something along the lines of the heat exchange system would be ridiculously large?

My AC that is lower in the housing complex literally heats your AC that hire on the termite hill? Thats almost the definition of getting roasted..