A mismatch in expectations and general incompatibility will do more damage then any imbalance in any system.
However: An imbalance coded into the system can
create a mismatch in expectations or a general incompatibility.
Because we've literally heard from folks on here--IIRC Charlaquin, or possibly Steampunkette?--how their innocent pursuit of thematic stuff for a character
caused them to become stupidly powerful (e.g. taking Incantatrix because they thought "oh cool, my character loves tinkering with spells, this perfectly expresses that!"), while other players at that same table, equally innocently pursuing thematic stuff for their characters,
caused them to end up being far weaker and far less able to contribute.
This is part of why it's so dangerous to blithely dismiss system as being totally overwhelmed by external factors. The system itself can spawn the very "external factors" you're claiming are always more important. Hence, they aren't
always the most important factor, even if they are the
direct cause. Sometimes, the indirect cause is more important than the direct one, just as (say) "lung failure" was the
direct cause of my paternal grandmother's death, but the most important cause was the asbestos she was exposed to as a young adult, which gave her mesothelioma in her later years, which resulted in eventual lung failure.
This does not mean that "perfect" balance is attainable--I don't believe it is, and as I said previously I don't expect it of anything. But it does mean that "well mismatches in expectations are way more important" can blind us to how system actually does play a part,
some of the time. It can't be all of the time, because if it were that would mean TTRPGs were mind control. But it is false to assert that poor balance is never the reason why problems happened,
even when we recognize that the most direct cause is often expectation mismatch.
I mean, we can literally see how the design of 3e did exactly that. The designers
expected people to play the game precisely as they did 2e, as though all the benefits and detriments were unchanged. In fairness, early on, most did just that. Then people started playing the game actually presented to them, and rather a lot of problems appeared.