While I've almost always read bad online reviews for McDonalds in the US, I think the experience might differ a lot in other places around the world, for context on what I'm about to write and why I think many people think the price is the worst offender.
Believe it or not, I've always had a good experience at any McDonalds I went to service wise. Over on this side of the pond they're seen much more positively in general, I think. Their food is pretty good by fast food standards, and it's way more consistent quality wise than pretty much every other fast food place. In the last few years they've also turned most of their restaurants from the typical fast food restaurant decor to cozier and actually comfortable places to hang out in, which is pretty cool.
BUT, the prices just don't make much sense anymore. The price for a basic menu is currently near the price point of an actual restaurant meal. At shopping centers over here there's usually always a couple of "somewhat fast" local food restaurants - I don't know what to call them, kind of an in between between what you would call fast food and what you would expect from a "slow" fancier restaurant - and it's actually become cheaper to get a normal menu there than at McDonalds. The one big attractive of McDonalds and fast food in general used to be price - but if it's currently cheaper to get a full plate of great much better food, why would I ever go to McDonalds unless I'm really in a hurry? They seem to be counting on people still going there out of habit, but I think there's not nearly as much of a fast food culture here, so I've started noticing that McDonalds around me have much fewer attendance than they used to.