At least with an iron gear, you can use magnets to help remove iron grinding from oil. Not so with bronze. I think part of the issue with roller cams is longitudinal cam thrust. With flat tappets the cam lobes' tapers compensate for each other and reduce the "natural" thrust generated from a spinning shaft and the valve train pressures. Rollers have "flat" lobes, and probably need thrust plates on the front of the cam. Not a big deal for factory roller engines, but for engines not designed for rollers, there other remedies are usually needed.
