I have a Magnum 36 XLS that runs very well. I have not noticed any roughness or other problem with the bearings. I don't have that much run time on it, only flying the plane in Classic at some contests. Maybe bearing replacement will be necessary soon.
On another engine, an ASP 46, the front bearing felt rough and gritty after some break-in on the bench. A local bearing supplier had them so replacement was not a problem. I guess it is hit and miss with the Magnum/ASP engines regarding bearing quality.