Mainly that it only benefits giant instances. That's a convenient feature for people on a tiny instance following users on mastodon.social, but adds friction for users on mastodon.social who want to follow users on a tiny instance.

I haven't looked at the implementation, but I'll assume it's flawless. The issue is that it doesn't address the broader problem. Like the "official" iPhone app that has a giant, colorful "Join mastodon.social" button above a transparent "Pick another server" option, it serves to push people toward m.s and away from a good federated experience.