I think the coolest thing about DDG are the 'Bang' searches: https://duckduckgo.com/bang
I'd use if the typical browsers didn't already have that feature built-in.
EDIT: It occurs to me that they don't on mobile, so I guess it's useful there.
Also, even if they have thousands, I don't think it would be too hard to find a site they don't support. Who knows how long they'd take to add the keyword on request, if they decide to add it.
Pros and cons. No solution is perfect. I guess I'd find DDG's feature more useful if I relied more on my phone for such searches or used random computers I can't setup for myself.