I’ve flown across a lot of the passes in that area, mostly in gliders it has to be said but we are quite sensitive to the weather.
The major issue is that the alps along the French/Italian border normally delineate what’s called a “climatic barrier”. The weather can change drastically over a very short distance as often dry air from the mountains meets damp air from the Po valley in a convergence, which can trigger meaty storms. On one side it’s 12,000’+ cloud bases and 10km away there is 8/8 stratus < 1,000AGL.
Often the conditions on the Italian side are the limiting ones as it can get pretty murky when there’s not much wind and haze gets trapped under an inversion.
This is from the Dolomites looking south from 14,000' but it gives you an idea:
Here’s one of the Col du Mont Cenis in good weather but you can imagine it full of cloud to the east like the above picture
