I think there’s no 26 disadvantage when it’s non technical. The issue is when it gets rocky. However I think most courses aren’t that rocky.
TBH, the only place i’d prefer a 26″ would be on tight (really tight) technical climbs. Everywhere else a bigger wheel will roll over more and roll faster. Non-technical, Rocky, Technical, downhill, fireroads and so on.
And experience tells me that courses got less rocky in the early/mid-noughties and since have been getting progressively rockier since about 08-09.
It’s not all rocky though, they just seem to be fireroad/rocks/singletrack/rocks rather than rocks/rocks/rocks which it used to be. (obviously not all courses, just the hard/fun ones)
FWIW theres a course near us where the organiser/landowner spent a couple of weeks dragging rocks from a small quarry on his land to insert/bury/stack onto a couple of 300+m sections of trail that are used in the race. Just to make it rockier.
We already had to race across the bottom of the quarry. 😕