Vegetarians rejoice! You got McDonald's to replace a saturated fat (tallow's mostly stearic acid) with a non-animal-based polyunsaturated oil.

Keeping a polyunsaturated fat at deep-fryer temps for hours (or days) is a good way to produce trans fats, which we humans have no way to metabolize. On top of that, McD's went through a number of formulas, trying to make their fries taste the same. It still hasn't worked. I'm sure whatever they've used is either tested safe or GRAS (generally regarded as safe) - but what happens when it's kept at high temp for long periods?

FWIW, some of the best fries I've ever had were at a flat mate's parent's place in Caen - cooked in duck fat. His dad apologized profusely because the "fat was too new", but they were good enough that I remember them nearly 50 years later.

Aaaand that brings us to the "French Paradox". When Keys decided to demonize saturated fat, he seems to have gone looking for evidence for his hypothesis rather than trying to disprove it (reverse of the scientific method). If data from some nation doesn't fit your hypothesis, exclude it, call it a 'paradox'.