We use unit tests to check our logic on smaller microflows.
Most larger flows can be broken into smaller testable units and this can help give you confidence that your larger flows are working correctly.
When testing APIs you could break out the part that uses the response and create a series of tests that pass in dummy test data that simulates various response scenarios.
As well as unit testing look at browser based testing such as ATS and Selenium. We use these to check the end to end process, mocking a user entering data and clicking buttons.
If it is worth to do unittesting heavily depends on the use case. In general the need for unittesting is lower than in high code applications, because a lot of issues you would normally do unittests for in high code are already covered by the platform. There is no need to unittest Mendix, only test the logic you created yourself.
In general: if you have a lot of complex business logic in microflows, it may be worth doing unittests on microflow level. The appstore has a unittesting component for this. Sometimes it is also enough to just create some testing logic yourself in the form of a microflow or grid that is present for testing purposes only.
Automated testing is also possible via ATS or selenium based systems like robotframework for example. This may require quite an effort and it also depends on the situation if that adds value.
I hope this helps.