This is my experience too. In my office I look after 40 trashcans, 4 are duds which Apple refuses to acknowledge. Luckily 2010 and '12 are still amazing machines which after upgrades work better than D300's trashcans anyway so I was able to reuse old gear.
I am able to demonstrate in production environment that if I clone "the dud" to a known working machine there will be no issues and vice versa.
Similar here. I’ve worked in several universities, colleges and businesses and have kept in touch with my replacements.
Off the record, 3 different techs who work at apple authorised resellers have stated to me in private the same.
I have 2 D300 computers. One is a very early one that’s never had a single issue. It’s currently running Sierra. My other computer that I purchased new from apple started failing after 9 months. Apple essentially replaced everything except the psu and io header with no effect. In December 2017 after significant effort on my part they did replace the interconnect/logic board (the circular one). It’s been powered on constantly more or less with absolutely zero freezes or crashes.
In order to get them to replace the board I had to film it crashing with minimal hardware connected.
I have emailed and written to apple regarding this issue. Rather annoyingly the response I received was to take this up with their support. I did write back stating I’d tried that multiple times but received no further response.
I know a few people who’ve struggled to get apple to fix the issue. In 2 cases, the users claimed they ended up buying refurbished machines from apple, carefully removing the circular board and swapping it for their own which fixed their issues.
Appalling that people have to resort to such extreme measures as this and the external gpu solution. This isn’t why I pay the apple tax.