I think it is a very fraught topic. I personally believe that in...

I think it is a very fraught topic. I personally believe that in the main (and yes there are always exceptions, either system or idiosyncratic) it is safest to assume market efficiency most of the time (almost always), but there are times when it is clearly safest to assume the opposite. Those times are when the vast majority is very much in agreement. The obvious times this happens is at the extremes, at times of euphoria or fear.

There's a caveat. Much as information is now widely available & thus driving "efficiency", it is also driving consensus & group-think. The problem is that while markets may reflect information semi-efficiently, they reflect sentiment very efficiently. So what looks like "markets pricing efficiently", may actually be long cycles of consensus.

Ultimately then, best I can see, is that one should attempt to bat against entrenched consensus themes, as long as the duration of that theme cannot bankrupt you. But even more, that one should swing against the extremes of panic or euphoria.

What has not aged is Templeton's observation that "Bear markets are born in pessimism, grow in scepticism, mature in optimism, and die in euphoria."