This huge OS project, Magento, have TONS of guidelines, most about decoupling,, it has an extended MVC structure with even more layers than those 3.. All good in theory, guess what.. Guidelines is not followed..
Changing order of two blocks in the view breaks business logic.. So much for decoupling.. You would not believe how many hours I've spent debugging this..

And I can't believe I've dedicated 12 years of my professional live to this platform..