That's true but there are degrees of "turbulence", degrees of "mountain waves" (which exist anytime there is
any wind flowing over ridges), degrees of "rotors" (comment as previous), and degrees of "wind shear" (which exists anytime one is climbing or descending with any wind present whatsoever).
Before I did my first flight over the Alps in 2004, I got all the usual dark warnings about "killer mountain waves". I think most of them were from armchair / pilot forum pilots. But knowing no better, I first flew to Wangen-Lachen in Switzerland and sat there for about 5 days, eating £20 sandwiches, waiting for wind-free conditions. Then we did the flight... being VFR, Zurich would not (completely pointlessly, as far as I can tell) let us into their FL130-base Class C so we flew most of the way across at FL129, getting nice pics like
this, and sure enough it was smooth. But other flights followed, and the other week I got
this pic from about 5000ft above the terrain, with I guess about 20kt wind, and it was smooth too.
So I think reasonable rules can be applied.
What you do not want to be doing is crossing mountain ridges while flying
at your operating ceiling
Then, any downdraught is going to force a descent.