Check out this thread in the Commuting forum, which is about the XT Shadow: http://www.bikeforums.net/showthread...y-worth-a-crap

I just switched my Cross Check from the 2011 factory setup (DA 9s bar-end, Tiagra 4500 RD, 11-25 9s cassette) to DA 9s bar-end, XT M772 Shadow RD, and an 11-34 cassette. The XT M772 RD definitely seems, as "surreal" said in that thread, "finicky." I've set up a number of SIS RDs, mountain and road, over the years, and the M772 seems considerably tougher than others to get tweaked just right.

Perhaps your mechanic is accustomed to more forgiving RDs and blaming the gear for his inability to get it just right, invoking no-longer-true DA compatibility problems.