In an earlier post titled Use $this->fail() with Mockery::on(), I explained the challenges of debugging a failing test with the closure passed to Mockery::on(). Instead of returning false, I opted to use $this->fail() - which seemed like a good idea at the time. After all, I was doing my test, then failing with a useful bit of information. (Previous to this, it would just say that you don’t have a matching handler for this assertion, which was really confusing).
When you have a more complex assertion you need to make on the parameters of a mocked object, you might use the Mockery::on() method. It can be hard to tell how this fails, though, because if the assertion fails somewhere, the message is confusing - it basically says that there was no matching call to that method, which is technically correct.