I agree, but I think “both sides” has merit when one side is trying to position themselves as champions of reason across the board. It simply isn’t true for any major political ideology (in America, anyway).
But you can use reason to work out that if people with different subjective values use perfect reason to choose their political opinions they will often end up disagreeing.
So it isn't true for anyone (in this universe, anyway).
Edit: not to say that pointing out that "both sides" have some sort of mistake is never valid, it's just that almost every time I see the argument it's from someone akin to a flat earther, or climate change denier, or an anti-evolutionist, who really doesn't have the facts on their side. And it's incredibly frustrating that they just expect me to respect their opinions just as much as I would respect someone who's talking points hadn't been debunked over and over already.