A great question. Honestly it's a very natural fit for me, so I'm probably not very well qualified to answer.

There can be a lot of code repetition. To me, that's not a bad thing. I've seen people to themselves in knots trying to DRY some common code, just to later have to undo it all because a requirement changed for only one of the code paths. And, as I said, I like my procedures to read like use cases.

I'll see if I can think of anything else...

You need to write more structure than if you had simply just put all the code in the web handler or whatever. You also need to be able to abstract all DB/API calls if you want to test easily, as mocking things you don't own just leads to pain later on.