The worst issue with JS cryptography is that it's almost always pointless, as it is here. Your users can't trust that you'll protect their secrets, because any coercive adversary who would ordinarily operate by copying those secrets off the server directly will instead simply force you to host a backdoor that breaks the crypto.

That's the worst problem with JS crypto, but we haven't enumerated all of the problems on this thread, nor does my (old) post on our website do so either.