There is no structural issue with the 6-bolt mounting standard. As it has been explained to me by folks that are much smarter than I, as regards boost hub adapters (using longer rotor bolts + spacer), the hub/rotor interface in a 6-bolt system is largely a friction load between the surfaces v. a shear load on the bolts.
Rotors themselves start to get flexier/weaker unless if they're made of thicker steel, use a two-piece design (aluminum core), or otherwise have a focus on lateral stiffness. The good news is these rotor designs also tend to take more abuse and, in my experience, make less noise (vibration).
Unfortunately, I could see Centerlock becoming the de facto standard - coming over from road bikes along with flat-mount. For the 2021 model year Shimano hubs have made a massive comeback from the lower end to mid-level (SLX) of performance mountain bikes and there's no reason to think that trend won't continue for 2022+ and in general, riders don't appear to give a sh*t if their bikes come with 6-bolt of CL rotors.
Jan. 7, 2021, 5:05 p.m. - Andrew Major
There is no structural issue with the 6-bolt mounting standard. As it has been explained to me by folks that are much smarter than I, as regards boost hub adapters (using longer rotor bolts + spacer), the hub/rotor interface in a 6-bolt system is largely a friction load between the surfaces v. a shear load on the bolts. Rotors themselves start to get flexier/weaker unless if they're made of thicker steel, use a two-piece design (aluminum core), or otherwise have a focus on lateral stiffness. The good news is these rotor designs also tend to take more abuse and, in my experience, make less noise (vibration). Unfortunately, I could see Centerlock becoming the de facto standard - coming over from road bikes along with flat-mount. For the 2021 model year Shimano hubs have made a massive comeback from the lower end to mid-level (SLX) of performance mountain bikes and there's no reason to think that trend won't continue for 2022+ and in general, riders don't appear to give a sh*t if their bikes come with 6-bolt of CL rotors.