There are obvious problems with bringing iMessage to other platforms (basically the standard for any proprietary platform).
But a few other issues are these various cryptographic ties to things like appleid etc, which are likely harder to deal with.
I believe old black hat demos from 5+ years ago when they found some protocol flaws were constructing the message content manually, but still sending them through the messages xpc service (which is afaik responsible for wrapping things in protocol goo)
There are other issues as well nowadays: all apple hardware for many years has been able to store keys in the Secure Enclave (or element? One is for crypto, one is for javacard, and I can never remember which), so there’d be a loss in security posture relative to apple hardware (I would put money on the SE/SEP being more secure than any hardware crypto wallet - it’s just you can’t unplug it, so you lose the very real security of an air gap)
