I have a requirement to test marketing cloud functionality and the only testing information I've found concerns sending test emails. In my experience, if something goes wrong with any kind of drip campaign, the failures happen silently and can be costly. It seems relatively easy to create functional tests that will trigger a journey for a test contact and send to a custom Salesforce email service address. I'm wondering if anyone is doing this and if not, why not.
There are a lot of testing possibilities in Marketing Cloud. In Marketing Cloud Email Studio there is "Preview & Test" and "Test Send". These testing scenario's allow you to impersonate one of your subscribers and send an email to yourself as if you were that subscriber. This will test among others deliverability and personalization. Marketing Cloud Journey Builder offers an out of the box testing functionality. After validating your Journey, a testing button becomes available. This allows you to test the entire journey, and all it's activities, while ignoring any waiting periods you defined. You can choose to send or not send the journey emails, based on your requirements.
It is true that you can make some functional tests yourself. The only reason I wouldn't set up a test like this, is that you will be polluting your all subscriber/all contact data with test contacts and the same goes for any triggered send data extension you would be using. This would require a clean up afterwards (or you keep working with polluted data), using API to delete this test contact from your MC instances (all subscribers, all data extensions).