Carbon wheels make a lot of sense on 29ers the larger diameter rim is inherently less stiff than its 26 or 27'5in counterparts, and the extra stiffness of a carbon rim contributes massively to the overall stiffness of the wheel. ENVE's M50s, a more trail friendly version of the long running XC rims, deliver stiffness in spades, but are tuned to provide a little bit more give vertically, so that you get the benefit of the stiff rim when you need it, but it doesn't beat you up the rest of the time. The M50s are 21mm wide, internally, which is about 3mm more than the XC rims. This translates into a marginally wider tire, but more importantly, more of the tire's tread on the ground at any given time. Between this and the rim stiffness, you'll get experience a cornering on rails feeling.