Now those are the two ends of the spectrum, large OEMs vs small shops. What happens in the middle?I think boutique builders have come a long way in the last 2 decades, particularly on Subaru flat 4s. There are three things here that I think people always confuse - Reliability, Longevity, and Power Handling. Built blocks tend to have equivalent reliability, superior power handling but at reduced longevity. Example: an F1 engine needs to be rebuilt every couple thousand KMs so it's longevity is quite poor compared to a street engine, but it MUST be reliable during operation to be competitive.
And this is almost always due to the fact that a modern automotive factory possesses superior tooling that is out of reach of boutique builders. There are exceptions, like when the aftermarket acquires an entire factory line from the manufacturer. A rebuilt SBC from 1969 might have lasted as long as the factory block from the same era because of the crude factory tooling, but also remember that 100k miles back then was considered very high mileage. You and I know this because we've both work(ed) in the automotive tooling business. Most people do not.
I also think there's an issue with people "oversizing" a built engine for their needs. They think "well if I'm replacing the engine I might as well go with the block that can handle 1000 hp" not recognizing material choices may impact longevity. You should always choose the right combination of parts for the application. Not having clear and concise goals is the main issue here.
HKS, Alpina & Brabus make & help develop some OEM parts but also sell built engines. How would their longevity compare?