I think 1-on-1s can be "unreasonably effective." In one group I managed, I implemented 1-on-1s and watched year-over-year attrition drop by ~80%. I thought they were so effective that I implemented them in my marriage and saw great (albeit, not directly measurable) increases in marriage satisfaction. If nothing else, my wife enjoyed them enough that she is now the one who insists that we have a "weekly connect."

With that said, I think that almost all of their effectiveness comes down to who's leading it. When I first become a manager, I assumed that I would probably suck at it, so I read everything I could on how to be a better manager. That was especially true when it came to leading 1-on-1s. But, when I've worked with managers who just didn't care for them, or didn't care to learn enough about how to do them properly, they were invariably a waste of time. So, despite my own experience, I don't begrudge anyone who thinks 1-on-1s are a waste of time. In many (most?) cases, they are.