Given that brains are fundamentally governed by the same physical laws as everything else, there shouldn't be anything about them which cannot be replicated in some way by something sufficiently capable of emulation of their processes.

That's not to say it's simple. Just that brains obey the laws of physics, and as long as that's true, they should be able to be replicable.

Unless your contention is that brains are somehow able to operate outside the constraints of the laws of physics, in which case we're going to have a fundamental difference of opinion as to the nature of the universe and whether things with brains are particularly special.