The reason you don't see a lot about the long term reliability of readers is eInk devices have only been available most places since 2006 and with upgrades in technology many folks have tended to upgrade early even if their old device worked just fine. I've used eInk devices since 2007 and all of my devices purchased over the years (six total) still work fine. Keep in mind that current eInk screens use a glass substrate that can be fragile and need to be treated with some care.
Pocketbook devices support doc and docx formats, but to what extent I don't know.
Onyx Boox devices support doc, but I'm not sure about docx. Both brands support ePub as well, but can be harder to get in the US than a Kobo or Nook.