As max.hirschhorn@mongodb.com said well, "assert.commandFailedWithCode() should always be preferred over assert.commandFailed() because it is possible to write the input parameters to a command incorrectly where it fails with BadValue/TypeMismatch/FailedToParse/etc. and means the test isn't exercising what it intended".

assert.throws has the same problem and assert.throwsWithCode should be used instead.