REST APIs are nice, right up to the point you need a transaction... three things need to happen in the correct order, and all succeed or are rolled back. If the API is too granular, then you end up pushing this logic to the client, and that turns into a disaster.

I'd personally rather go back and extract the smallest possible functional API, rather than build a massive API upfront. You don't really know where the transaction boundaries are as you are developing, and it is easy to expose an API that not really possible.