It is indeed bitchy! I'd say simply having different scaling needs may not suffice to isolate services, especially if all the other parameters I mentioned stay

It is indeed bitchy! I'd say simply having different scaling needs may not suffice to isolate services, especially if all the other parameters I mentioned stay the same. Even so, some cases for needing to isolate purely based on scaling - in my experience - have been when the scaling requirements are drastically different, with completely different geographical needs, scale ceilings, etc. We try to keep authorization within the system itself, with additional hardening on the service mesh/container/vm/network level as needed. I don't know that this is the right way to do it, but it's worked for us thus far. Biggest problem with isolating services is that it reduces predictable errors by giving you smaller chunks you can reason about, but raises unpredictable, unreproducible (aren't those a bitch) errors down the line because the overall complexity has grown.