[jillesvangurp]

There are some nice studies on correlations between metrics (cohesiveness, coupling, complexity, levels of indirection, etc.) and maintainability. Basically anything that scores poorly is inherently hard to understand because it induces a high cognitive load.

The reason what you outline is bad is because they each impact these metrics. Bypassing layers creates a more tight coupling: you are basically putting code in the wrong place. This also makes the code less coherent. The two go hand in hand. And then you end up doing complex things like reaching deep into layers (which violates Demeter's law).

Anyway, MVC style architectures have always had the problem that they invite people to mix viewer and controller code and logic. And once your business logic mixes with your rendering logic, your code is well on its way of becoming yet another UI project that fell into the trap of tight coupling, low cohesiveness, and high complexity.