I-40 in the winter is mostly good traveling. It's just that when the snows come, it can be light or it can be horrible. The worst is when the weather goes bad (and it can be forecast for a light dusting but turn into a raging blizzard) things can be really bad. Nobody wants to spend half a day stranded on the side of a full stop interstate in the middle of nowhere in freezing weather with 12 inches of very wet heavy snow piling up around them. So, although your odds are only about 25% that you may hit really bad weather, should you, you may regret not taking the southern route.
Now, assuming you make it on I-40 with no incidents, with clear weather, no snow and bright sunshine - you run the risk of some fool in their vehicle barreling down (and I mean down) I-17 or paying too much attention to their smartphone and WHAM... Anyone who's been stuck on I-17 during a major accident knows what I mean. Once again, the odds are small but if it happens to you, you probably will regret not taking the southern route.
I personally have no issues as I'm set up for wilderness camping be it in the woods or lane 2 of the interstate. I also have the possibility of hitting the Forest or Fire roads to get around problems, but a larger RV is at the mercy of DPS & ADOT.
It's just a gamble; risk vs reward. The odds are in your favor that weather and road conditions wont be an issue, but if they are, just be aware it may not be enjoyable in the least.
1994 Itasca SunDancer 21RB - Chevy G-30 chassis.