I've absolutely no clue about computation but I have been told quite a few times that it doesn't matter how the brain works if you accept the Church–Turing thesis. It doesn't matter insofar as "everything computable is computable by a Turing machine." But as far as I can tell, that is purely theoretical. Theoretical in the sense that it works but that it could be unusably inefficient. If our brains make use of some novel physical processes, if indeed general intelligence demands (Church–Turing–Deutsch principle) to be run on quantum computers, then we might still be able to 'upload' ourselves into some crude mechanical device but it won't be efficient. Substrate-neutrality is likely factual but might be inefficient.