That's an issue I have, too much overreaching on the part of a director. I've had it in wind ensemble and I've had it in big band. My personal philosophy is to pick music that a group can play about 80% correct on the first run through. That's a good rule of thumb to judge the capability of the group on that particular music and also to pace the remaining rehearsals. I hate to get into situations (and it's happened all too often) where the group and leader are scurrying frantically trying to get it all together when the director has just shot himself in the foot. Too much stress and not enough music making. I'd rather take my time in a non-tense atmosphere refining the musical expression than sinking frantically still working on technical stuff with a performance imminent.