I was always a big proponent of openAPI. But the reality is: the client generators are often times lacking, spewing out shitty code with ages old dependencies. You sometimes have to make weird mental acrobatics to get your API reality modelled.

Then, not directly related, you have the problem of each service having to have their own client. Maintenance doubles.

We're going down the graphQL federation with "one graph" for backend services path now. Surely not a holy grail but way better than openAPI.