Over a longer time (>5yr) frame I've seen a lot of value in the Build option. Like CRM? Just buy one with a good API. Billing process? auth? Build. Annoying up-front but years later they've both shown to be cheaper and flexible than the Buy options.

Maybe the line for me is how dependant we'll be on the service, and how hard to switch? How much ownership of the feature/service does the company need now and in 10 years.

One startup I know used an auth-provider-service. It missed on some feature they needed so they had to hack a workaround and now also it's expensive.