Not sure what the "hard to find" colors are, although Sennelier has so MANY, I don't doubt some of them cannot be found anywhere else.
I've not had the massive crumbling problem with my Senneliers; minor ones yes, but then I'm a very light touch AND, I keep some of the rock hard ones (I LIKE them!) on hand for doing detail and/or line work. Very handy that way!
That said, I've supplemented my soft soft collection with Schmincke, which I like. But as far as I'm concerned, no one makes the luscious high key colors that Sennelier does, and I'll continue to buy those.
I did branch out (just for the halibut) and bought some Terry Ludwigs and these taupe - brown neutrals are exquisite and so creamy I want to eat them.
I suspect to some degree one's choice of pastels is almost as emotional as it is a $$ or color consideration. Some just feel better in your fingers!