I second that. There is a mile difference between the sorry excuse of a burger that’s called Big Tasty and McCrispy in the Netherlands versus the already way better proportioned and fresher one you get in Germany, up to the better ones in Italy.
Besides the bun, it is noticeable in every part. The amounts and quality of the sauce, vegetables, and meat. And finally how the burger is presented.
So if this difference can occur within 1000km of each other in the same continent, I fully accept that it is even more varied in the whole world.
Maybe your sample size is too small? I've lived close to the NL/D border for a while and the McD quality was indistinguishable on both sides of the border. The variation between restaurants in the same country and also between different days/times in the same restaurant was much greater than between countries.
Thatis, if you happen to go to a random McD in some country and the big mac was great that day and you go to a different restaurant in a different country on a different day and the big mac was bad, then that difference has likely least to do with them being in different countries. It's not like they actually use different recipes.
Okay, granted maybe it is. In NL it is mostly in cities (Amsterdam, Utrecht, Rotterdam, Dordrecht, Lelystad), in Germany with smaller places, and in Italy only in touristic places like Siena and Genoa. So maybe it is just a problem with McDonalds in Dutch cities.
Besides the bun, it is noticeable in every part. The amounts and quality of the sauce, vegetables, and meat. And finally how the burger is presented.
So if this difference can occur within 1000km of each other in the same continent, I fully accept that it is even more varied in the whole world.