It’s a fair point, “meta bad” is poor discourse. The most prevalent concern I’ve seen is that allowing federation to Meta is setting the stage for another Gtalk-XMPP style conflict.
In effect, when a party has such a disproportionate user base, they can use that to dictate terms on the evolution of the protocols that underpin a platform.
Here’s a write up by someone who worked on XMPP and Gtalk who puts it much better than I could. Article