OKRs are just a tool. They can work pretty well if you have competent people and apply them correctly. But, that can be said of any other project management system, goal management system, gamification system... Some are worse, some are better, but most have books written about them that heavily feature survivor bias and correlations/causation bias.

The OKR book assigns most of Google's success on OKRs. Assuming there is an alternate reality somewhere, where Google chose SMART goals in that critical point in time. In that timeline Google of today is one floor below Yahoo in some random Verzion corporate building.

I haven't seen a goal management system yet that I cannot destroy with bad management and misaligned incentives.