What I do find, and I can think of a few recent examples is threads that spiral out of control because it turns into a debate where we need a right and a wrong answer.
When you remember that we do this for much lesser issues, like playstyle choices, "damage on a miss", the content of D&D art and implied setting elements, and... just about everything else, this comes as no real surprise.
The big issue is when people start to take opinions and view them as objective truth. Sometimes that's the poster's intent, but most of the time they're just stating what they think is correct. I know sometimes I get accused of this because I sort of have a direct writing style. It's caused me to add more "in my opinion," or "just the way I see things." And I hate that because with a very few exceptions, everything anyone posts here is just their opinion, but I'd rather engage with people than have them think I believe they're wrong.
With respect, unfortunately, it gets more complicated than that, which is why good non-fiction writing generally leans on telling the audience what you actually mean. Leaving personal preferences phrased as objective assertions is sloppy. The more assumptions the author forces the audience to make, the more likely some of them will assume wrong, and the less the author can blame the audience for the miscommunication
But, then people get weird. Having
phrased it as objective, folks will then
defend it as objective, even if they later claim it wasn't intended that way. Making assertions frequently has a subconscious emotional impact on the speaker - we react poorly when we are told we are incorrect, and will get defensive to save face, even when we know we are incorrect. A whole lot of head-butting happens because neither side can allow the other to have the last word.
On top of that, there's been a larger-than-you'd-probably-expect number of folks over time who have needed moderator action who persist in asserting objective truth on some of this stuff. So while you may feel that, "everything anyone posts here is just their opinion", the audience cannot reliably assume authors think it is just their opinion.
So, I know repeating the IMO, and 'For my table, but YMMV" is a chore. It is a really helpful chore in the long run.