I’ve ridden a few cx events on my (26″ wheeled, mtb tyred) mtb:
In the dry – apart from the obvious disadvantage of 26″ v 700c dia wheels, I’ve been reasonably competitive against my own club members (on cx bikes) at the lower end of mid-pack.
In the wet/mud – mtb tyres are a huuuuuge disadvantage. Against the same club members I have to pull out all the stops to avoid being lapped over 1-1.5mile laps. Although better, I haven’t found that mtb specific 26 x 1.35 cx tyres make a significant difference in wet/mud either. However, on a 29er with ‘std’ cx tyres I’d expect to be back amongst my club mates again.