I don't know if Huawei firmware has backdoors, but I can tell you I've looked at it (when evaluating their network switches) and it's a humongous mess of NIH. They even wrote their own SSH server and userspace IP stack (but still use the Linux one internally). It's rather buggy; simply scp'ing a file off of the switch leaked memory leading to a crash and reboot. It's just very poor quality.
Backdoors or not, I have very little doubt their equipment is full of exploitable security issues.
In the UK we have an agreement with Huwawei and access to firmware, which can be bit-for-bit compared to what ships. We don't for other manufacturers. But guess which one we're told not to trust.
Backdoors or not, I have very little doubt their equipment is full of exploitable security issues.