I'm so sick of "senior/lead" developers pretending they know how to write tests and ending up with these unmaintainable test suites, full of repetitions and incomprehensible assertions.

You should take some time to learn from your mistakes instead of just continuing to write the same shitty tests as usual!!!

Every time I arrive at a new team I spend weeks just trying to understand the test suites for what should be fairly SIMPLE applications!

UNIT TESTS SHOULD TEST UNITS OF CODE!
If your unit test tests seem to be repetitive, they are not unit tests. Repetition is expected in integration tests, but that is why those are usually DATA DRIVEN tests!!!