First thing first, i want to clarify what i'm about to say: i'm not saying that no matter what non-tier A classes shouldn't ever be played and whoever plays a gunslinger should be kneecapped, that's not what i mean. (whoever designed the gunslinger should be, but this is an hyperbole and a joke, not an actual opinion)
So! the community takes great pride in saying that every build is viable and one can't just win the game in character creation; the second sentence stands true, but does the first? partially, in my opinion. it all depends on the encounter's difficulty!
moderate? yeah literally everything will do, low severe? now some classes/builds start to struggle and it's preferrable to not even bring them to the table; high severe/extreme? only really good builds actually work and system mastery is required.
and this isn't inherently a negative!, but i'd argue that saying: "oh every build is equally viable" is deleterious to the community. sadly in an extreme encounter i think that a gunslinger would never ever find space, and that everyone would prefer a cleric or a bard over a wizard or a psychic