I once read in a mountain flying handbook that in the mountains, winds over 20 to 25 kts at FL100 are a no-go
Depends on how high you are above the terrain, on the wind direction relative to the lay of the ridges, etc.
If you cross the Alps in the middle, say via RESIA (basically via Samedan, N-S track), at FL180, then you are about 8000ft above most of the terrain.
If the wind there, blowing N-S, is 40kt, then you can expect (very roughly of the order of) +/- 500fpm up/downdraughts. If FL180 is
at your operating ceiling, you are in for an exciting ride
I would not do such a crossing with winds above 20kt.