Any bearing can fail. I have 550's with 20-30K on them and head bearings are still good. When the balls rest in the same position on the races they wear a "pit". Roller bearings will do this too. Takes a bit longer with a roller because the bearing surface is larger. We use the same style of roller bearing in airplane wheels. If the plane is parked outside and stationary for too long. The wind elements shake the plane but the rollers don't get repositioned. Then they wear or "worry" a groove in the race and have to be replaced. Same failure as the ball race in the SOHC4 headstock.
There is no maintenance schedule stated for the head bearings. But, anyone taking apart an old bike has seen the grease dry out and congeal. Bearing contact pressure then pushes out the grease from the contact area, leaving metal to metal contact and subsequent wear. Once a small pit occurs, the balls register in the pit, and the problem exacerbates as new lube doesn't get pushed around much.
So, I speculate that it is largely a grease failure and lack of maintenance that caused your ball bearing issues. (I haven't seen your old bearings, of course.)
The balls type bearing "notches" much sooner if the adjustment pre-load is not kept up. The preload distributes the load to all the balls rather than just the ones at the top rear and bottom front. Was this where you found the notching on your old bearings?
Not only does the grease need to be replaced periodically but the pre-load needs to be readjusted occasionally, too, in order to distribute the pressure loads.
Any mechanism that is abused can fail.
Leave your tapered bearings loose, and deprive them of lubricant, and see if they last 10K miles without getting "notchy". (Don't really do this if you wish to avoid early replacement.)
I did put tapered bearings on a Cb400 a while back (Honda wouldn't give me a better deal on the ball type, and I didn't have to wait for delivery on the tappered bearings they had in stock.). Maybe it was the kit vendor I used (forget which it was), but I certainly didn't like the lack of water seal it gave for the lower race. Oncoming dirt, air, and water would easily force it's way in past the open end and wash out the lube. I had to adapt/make/shim a rubber seal to give it long term viability. This particular bike was neglected and the original ball races were quite notchy with the deepest wear at the top rear and lower front. Yes, the pre-load was quite loose and probably had never been readjusted after the time it left the showroom floor, and 25 year old grease, to boot. There was also some accident damage, as well, with the turn stops bent up. P.O.s weren't very kind to that bike, even though it had low miles.
The bike was much more stable and a nicer ride with the new bearings. But then, I didn't change new ball bearings for new roller bearing, either. So, I can't testify that roller bearings outperform ball bearings in an even comparison.
Lastly a bit of a correction. I said the roller bearings were "far, far in excess". I meant the wording in the following sentence to imply that the stock ball arrangement was only "far in excess". Certainly the rollers will take a larger load than the ball type. But, the application requirements are well below the capabilities of either.
The fable of Noah and the Ark should show the while having plenty of water is a good thing, a lot more may not be of much real benefit.
Thanks, for noticing the humor.
Cheers,
