I'd argue that even the crazy ghc features are mostly trustable. Virtually every language extension and even a lot of the optimizations are explained in papers

I'd argue that even the crazy ghc features are mostly trustable. Virtually every language extension and even a lot of the optimizations are explained in papers - including design tradeoffs, proofs of correctness and interactions with other extensions. That still doesn't mean that it's a good idea to enable -XIncoherentInstances, though.