There are always going to be mistakes made when rebuilding engines. Not always by the same person but there are so many reasons an engine can not last that it's more important the person building your engine gives warranty on their work. I always put 5000k warranty on engines for that reason. No one is perfect so even the bad stories you hear may not be all that bad at the end of the day.
And usually if an engine can last more than 500km, the engine builder has done things correctly. If the engine builder stuffs up, you won't get 500km on an engine