I came across a good example of the interoperability concerns today when people boosted :petthex_javasparrow:しゅいろ:petthex_javasparrow: (@syuilo) | Misskey.io in to my timeline.
Here’s what that looks like on the original server:
Note the emojis in the name (above the @syuilo) and the wide array of emojis at the bottom, many of which are decidedly non-square.
Here’s how that renders in Mastodon:
The emojis at the bottom are missing (they’re emoji reactions, not supported by Mastodon, so that’s understandable). But note that the emojis from the display name are missing too, and this renders as :petthex_javasparrow:.