The game goes to a lot of effort to establish: an action economy (eg some monsters have multi-attack; some clases have extra attack; fighters get action surge; etc); different allocations of damage (a fighter can choose damage as a fighting style, or AC, or "to hit" with ranged weapons; using a shield to boost AC takes higher-damage weapons off the table; spells have a range of rather arcane damage expressions which presumably were put there for a reason; etc); different trade-offs for condition infliction (eg the basic shove attack precludes dealing damage; monks have to use a limited resource to stun foes; etc).
I think it's a bit unrealistic to expect a referee to reverse engineer all that on the fly. But in many groups I think it's also unrealistic to expect a referee to just ignore it - if the player of the monk is spending PC build resources to stun single targets, and the fighter is able to make easy Athletics checks to pull rugs from under enemies and stun them all with a single attack that consumed no rest-based resources, that is going to raise issues of balance/overshadowing at many tables.
In a mechanically simple game like HeroQuest revised(where all abilities are just free descriptors rated on a single numerical scale), it's easy enough for Robin Laws to give advice (I'm paraphrasing a bit) along the lines of "If it's a minor overshadow of another PC's particular ability, apply a minor penalty (-3 on d20) to the check; if it's a major overshadow, apply a major penalty (-6 on d20)." But 5e is more mechanically complicated, with many more moving parts (and yet, at least by default, only one category of penalty, namely, disadvantage).
I think this is what makes the request for guidelines logical. In this respect, I don't think 5e is wildly different from 4e (which has a comparable attention to the details of action economy, damage differentials and condition infliction).
EDIT: I just looked at post 50 - overlap with feats is another consideration that comes into play. Is a GM meant to be familiar with every feat on every player's character sheet as part of the process of adjudication? That's a big ask!