If you look at the pics posted above you can see why the needle bearing are junk. They have no bearing area. They are not even the width of the rocker.

And then there is the pesky problem of unit loading. Even if the needle bearings were the same width as the body of the rocker, you only have a very small area of contact between the bearing and the shaft. The fact that we HOPE the bearing rotates is of little consequence. Once you flat spot a bearing or the shaft, you are done, over.

And then there is the issue of of valve lash. On a HFT or HR there is no real lash in the system. With SFT or SR lifters there is a running lash. Think about a .020 run the pushrod has at the rocker! Most cams lash a bit looser than that. Now take that beating of the pushrod on the rocker (and then it is transferred to the bearing) and mutiply it by whatever RPM you want, and consider the destruction. At a 2500RPM cruise, you are just hammering the bearing to death. What about 5000 RPM? Or 7500 RPM? Or, if you have the good sense not to but do it any way, 8500 RPM? Failure is the only option.