anything is rebuildable, the issue is the cost of doing so. If it costs $300 in parts, $50 in shipping, and $450 in labor to rebuild an engine, is it feasible when a brand new engine can be obtained for $595?
Usually the benefit of rebuilding it is the labor one saves when they do it themself. However it they have problems after the rebuild then what have they achieved? Also if machine work was needed as part of the labor cost, one may still have some labor cost in rebuilding it so the savings may not be as great as one thinks they will get.
Another thing DIY need to realize is that shops who do the work, know what quality they put out and what their be-back rate is. They can factor that into their prices and day, but when a DIY is involved that shop may not want the machine work job due to the risk of be-back if the DIY makes a mistake, and the amount of their time that ultimately ends up in it when the DIY starts asking for free advice.