The aboot implementation in the Qualcomm components in Android before 2016-07-05 on Nexus 6P devices omits the recovery PIN feature, which has unspecified impact and attack vectors, aka Android internal bug 28822677 and Qualcomm internal bug CR804067.
https://source.codeaurora.org/quic/la/kernel/lk/commit/?id=fa774e023554427ee14d7a49181e9d4afbec035e