Although I agree it is risky, emoji are unicode characters, just like any other unicode character. If, and that's a big if, the programmers do their job right, it shouldn't matter if you use an emoji or a random kanji. It's all just another character. That said, I don't trust programmers enough to run the risk. Your password might work fine on the website but then fail on the mobile app.

Someone else said "good luck on the desktop", but Windows actually has an emoji picker built right in. Win+. will bring it up. Another fun fact, usernames and computer names both support the full unicode set on Windows, including emoji. Some fun can be had with that knowledge. I haven't tried it on Linux or MacOS yet.