Path-based selection of tests, in every single CI system I have ever seen it implemented in, is wrong. TFA thankfully gets the "run the downstream dependent tests" which is the biggest miss, but even then, you can get the wrong answer. Say I have the following: paths a/* need to run tests A, and paths b/* need to runs tests B. I configure the CI system as such. Commit A' is pushed, changing paths under a/* only, so the CI runs tests A, only. The tests fail. A separate engineer, quickly afterwards (perhaps, lets say, even before the prior commit has finished; this could just be two quick merges) pushes commit B', changing paths under b/* only. The CI system runs tests B only, and they all pass so the commit is marked green incorrectly. Automated deployment systems, or other engineers, proceed to see passing tests, and use a broken but "green" build.

Since ever rarely do I see the downstream-tests requirement correctly done, and almost always the "successive miss" bug, I'm convinced for these reasons path based is just basically a broken approach. I think a better approach is to a.) compute your inputs to the tests, and cache results, and b.) have the CI system be able to suss out flakes and non-determinism. But I think a.) is actually quite difficult (enough to be one of the hardest problems in CS, remember?) and b.) is not at all well supported by the current CI systems.

(I have seen both of these approaches result in bad (known bad, had the tests run!) builds pushed to production. The common complaint is "but CI is slow" followed by a need to do something, but without care towards the correctness of that something. Responsibility for a slow CI is often diffused across the whole company, managers do not want to do the hard task of getting their engineers to fix the tests they're responsible for, since that just doesn't get a promotion. So CI remains slow, and brittle.)