Conclusion
Thanks for all the replies.
I did find out that Koyo are Jap manufactured, not long after making this post, so I'm not worried about using those now.
Agreed that most times it is not the bearings fault that causes the failure, I've pulled failed bearings only to find a bit of crud sitting between the outer race and the seat. Of course, this puts the out the alignment, causing wear and premature failure, hence the need for cleanliness when installing any precision part.
Also, I was recently enlightened to the fact that it was actually a certain type of seal, used in conjunction with the bearing application in this case, that used to cause the failures. A lot of aftermarket pattern seals were being recommended years back, but they have since been found to starve the bearings of oil. After a second hands on investigation with all my old engines and cranks, this does indeed seem to be the case, so I shall be using good Jap bearings, along with the correct type of seal and all should be fine and dandy.
In case you're wondering, the bearing / seal application I'm talking about here is with Yamaha 2 stroke engines, more specifically the two centre crank bearings and the labyrinth seal between them. The genuine Yam seals have raised intermittent sections around the edge of the seal that allow oil to pass trough and into the bearing races. The pattern ones have a raised, but solid area here. My mind tends to tell me that the pattern machinists must have summised that the raised parts were there simply acting as some sort of spacer, either that or they just didn't care about it's true purpose, as it was certainly easier or at the very least somewhat quicker to make them that way.
Superior Yamaha Type Labyrinth Seal
Inferior Aftermarket Type Lab Seal
So that's that sorted!
Thanks folks.