The only problems i had when running a SH based engine was the carbi sleeve seperating from the carbi, Apart from this the engines gave me good service.
So many factors can be a caused for engine fail. Usually its reving the crap out of it on the box when an engine is cold, i see this all the time.
Poor fuel ie poor quality oil, or not not enough oil within a given mix.
Bearings seem to be a big problem for engine manufactures, but again it all comes down to how a person treats his/her engine.