I've found the number one cause of flakiness in tests is from misuse of nondeterministic or highly stateful functions. This becomes especially apparent when you recognize a lot of nondeterminism comes from misusing datetime libraries such as momentJS, or from Math.random . Even integration tests against certain "eventually consistent" databases tends to result in flakey tests. Entropy and Time libraries are intentionally nondeterministic, so the use of these should be recognized when writing tests, and designing functional "cores" that can do predictable operations when given testable inputs.
Ain't no one got time for flakey tests.