Some people have good results with aftermarket parts. Others claim to have had failures. The problem here is that it is not possible to pinpoint the TRUE cause of the failure and perhaps the aftermarket kits get blamed out of convenience.
A couple of issues I did not address in my previous post are the governor gear and the balance gears. Both are known to fail on engines with high hours on them. If the governor gear fails, the engine can rev far beyond the 3600 RPM point and self-destruct as a result. Old balance gears can fracture and fall into the pan and make contact with the connecting rod. This can break off the dipper and that causes a lubrication failure that leads to engine failure. The gear can also end up causing catastrophic failure of internal moving parts which may lead to holes in the block, thus rendering it useless.
If an engine has 2000 hours on it, then every part in that engine has 2000 hours on it and is suspect. Experienced engine rebuilders have often learned the hard way about balance gears and governor gears. Just because someone gets away with not addressing those issues means that they were lucky. However, will that luck hold out until the next rebuild or will one of them come unglued and turn a good engine into scrap metal?