I agree with your general principle

There are of course exceptions, but they're very few and far between, so one should embark on a project that contradicts this principle only if there is some very good indication that it might be an exceptional situation. (For example, it might be that the idea really is too clever to have ever occurred to anyone, or there might be some systematic biases limiting its adoption, or it might be that everyone is in fact already doing it, only you've been oblivious about it so far. However, for most people, only the third thing is less than extremely rare.)

The factor I focus on when considering the potential for exception is money. If there was an exception would have given participants a way to get rich (or powerful or laid) then I assume that it would have been considered already. So I wouldn't even bother looking at stock market 'exceptions', for example. I would look at medical 'exceptions' so long as the standard variant was the one with more financial success.