The thing is.. that even if there are countries publicly rejecting this, once the infrastructure is in place and a backdoor exists due to it being enforced by some other country, how can you be sure it's not being used / exploited?

Even in the (hypothetical) case that the government is not using it (regardless of what they might say to the public), I wouldn't trust that this backdoor would be so secure that nobody else than a government could make use of it.