Anytime you set an indirect measure of success, system-manipulators will focus on the metric instead of what that metric was supposed to measure. My experience with OKRs is that teams and individuals with OKRs will:
A) Game the system. (For example, at Zynga, one team set an OKR to "examine frameworks for such and such," since implementation wasn't part of the OKR there's no way to argue they didn't examine). Individuals will be incentivized to pick OKRs that seem hard but are actually easy.
B) Disincentivize proper behavior. Engineers are valuable because they are not robots. They are thinking, evaluating, reacting people. The best engineers aren't those who meet your expectations, they're the ones who upset your expectations by inventing a side project like gmail that becomes a portion of your entire company. Valuable engineering can take many forms (assisting others, preventing security holes, cheering up cynical colleagues, standing up against bad decisions), and almost none of them can be directly measured.