I'd be hesitant to rely 100% on this approach (and from the notes it sounds like FB still runs the other tests - just less frequently in some cases), especially

I'd be hesitant to rely 100% on this approach (and from the notes it sounds like FB still runs the other tests - just less frequently in some cases), especially where the code coverage tools aren't guaranteed to account for unintended interactions between seemingly unrelated pieces of code. In theory, well-written code/tests shouldn't have those kinds of problems in the first place, but larger projects often don't deserve that level of trust (in my opinion). Still, I think it's a great idea for giving quick, "preliminary" feedback to developers - as long as the full test suite is still run periodically.