Because GX.sigma said: "It was impossible to hit him. For the rest of the campaign, he never got hit. It was literally game-breaking." From that I think it is reasonable to deduce that GX was relying too much on attack rolls to threaten the characters. If not, then why would the game have broken?
I would not conclude that GX was relying on attacks rolls more than I do, or more than I'd expect in a typical game. But evidently, they were relying to much on them for the situation at their table.
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Hah well anyway on this topic AoB and I seem to be on the same page.
I'll address both here, then. Not ever being hit is game breaking, even if you do not "over-rely" on attack rolls. This is because it would take a serious divergence for attack rolls to NOT be the primary method of reducing hit points. A DM would have to go out of their way to select monsters/NPCs that do not rely on attack rolls, and there are very few that do this. Even dragons, the titular creature of the game, rely primarily on attack roles to reduce hitpoints. So the other path would be to reduce the impact of combat as a resolution mechanic, which is another departure from the baseline. No problem in choosing to play that way, of course, just noting that given the wealth of intended play examples in the official products over all editions, combat is THE primary conflict resolution mechanic. And, again, within combat, attack rolls are THE primary damage vector.
So, there exists a whole subset of games were you can include plenty of non-attack roll based damage and effects, and STILL have being nearly unhittable destablize the game. And this includes that idea that the DM can just pick monsters to offset the issue of build choices, as that's still a disruption of the game by changing the baselines in which the choices were made to begin with. IE, why would I have expended effort to acquire a clock of displacement if the game was going to change to one of only fighting enemy spellcasters with save only spells because I did so? Further, having to make such changes could dramatically affect the planned scope of the game. For instance, if you are running Storm King's, and this situation obtains, what's your solution? Giants clearly "over-rely" on attack rolls to reduce hitpoints.
So, yeah, the assumption that the DM didn't properly make choices to offset an unhittable character is a poor one -- you have no idea what happened, and your insistence that they did something like 'over-rely' is, itself, assumptive of bad practice.