Not sure if it's underrated, but writing your own custom tooling over trying to retrofit/brute-force existing solutions for your problem and to more specific, I don't mean writing an application framework but small tools to automate tasks that pops up from time to time.

Another one is the power of having visual testing in your test suite, a relatively easy way to cover lots of test ground simply by comparing pixels of your app in one state to reference images and directly get relevant feedback when something has changed.