Good question, I've wondered about it too, they do share some saints with Catholics and Orthodox from the apostolic and patristic periods. Don't know about OO's but I I've read that Nestorians were treated like theological bumpkins for similar reasons.
>Of course Christian Europe would forever view the Nestorians as “false Christians”; as oriental bumpkins whose knowledge of Christ and doctrine was tantamount to paganism, and would even dispatch missionaries with the sole purpose of “converting” them to Christianity.
https://borderlessblogger.wordpress.com/2014/04/30/the-nestorian-legacy-aramaic-mongolian-christians-and-the-persian-church-of-the-east-2/