I think the difference is in cross-airline revalidation when all parties involved are Amadues carriers, not that Sabre cannot revalidate. For example, if you have a QR-157 document with QR, CX, and BA segments, in Amadeus the CX and BA segments can be revalidated, but not on Sabre. Sabre can definitely revalidate QR segments on a QR-157 document.

Given it is impossible to have an xONEx without Amadeus carriers (but possible to have only Amadeus carriers), that's why AA always re-issue to make sure the Amadeus carriers can see the latest copy of e-ticket.