> This all seems cobbled together and very prone to failures.
AFAIK it's not like FastMail has a crazy number of network-related outages, so overall it doesn't seem that "prone to failure". As with many things, it's a trade-off with complexity and costs.
I'd argue that often the CDN or transit isn't drop-in replaceable. So it's usually more than 2x the cost as one has to maintain two architectures (or at least abstractions). That includes the expertise and not optimizing for strengths of either, or building really robust abstractions/adapters.
AFAIK it's not like FastMail has a crazy number of network-related outages, so overall it doesn't seem that "prone to failure". As with many things, it's a trade-off with complexity and costs.