I don’t think it’s prudent for a system to disallow emoji passwords. Or to restrict various characters. I say: let the user do what they want to do; on their own heads be it if they do something that doesn’t work for them in all circumstances, and the end result is practically not much different from forgetting a password, which is normal.

In general, passwords are not treated as essential for access, and there will be recovery techniques, and the number of password resets or whatever required because they can’t type such-and-such a character any more or on this new device will be a minuscule fraction of the total. Resetting passwords and other forms of lost account access is typically not an exceptional path. From what I’ve seen, for business-to-customer businesses that don’t have some form of self-serve account recovery, “I’ve lost access to my account” will routinely be half your ticket volume.

In those fairly uncommon situations where passwords are essential for access (e.g. where it’s an encryption key), well, it’s still up to the user, and the user is somewhat more likely to be aware of any potential hazards in such fanciness.

Overall, I say: stop trying to be clever; accept what is set before you without asking questions on the grounds of compatibility. Let the user do what they try to do.

Maybe normalise Unicode; it’s a harmless thing to do and has the potential to improve compatibility slightly on very unusual input devices. (I don’t think I’d bother, myself.) But beyond that, I’m not sold on the arguments for restricting possibilities.