I think it's fine to mock out a call to an external service (e.g. your app making calls to AWS S3). But when you mock out too many calls to code in your own ap

I think it's fine to mock out a call to an external service (e.g. your app making calls to AWS S3). But when you mock out too many calls to code in your own app, that's brittle in my opinion. For example, say your testing the "Foo.new.().foo()" method, and you mock out calls to "Bar.new().bar()". You could make a breaking change in "Bar.new().bar()", say you go from returning "bar" to "BAR". The test in "Foo.new.().foo()" would keep passing while testing the wrong value. Now if you do that all over your application, you can have a nicely factored test suite that is completely detached from reality.