I think the big error in A/B testing is that expectations are quite often very unrealistic. Designers typically have a
reasonably good idea about what will work and what will not. Finding 'million dollar buttons' is rare. Of course a couple of percent or even 10's of percents of improvement is nothing to sneeze at. But thinking that by A/B testing forever you're going to make a shrub grow into a tree is imo not realistic. Aside from the detail that a continuously changing user interface is often in itself a barrier to sales.
Ironically, the companies that have benefited most from A/B testing were the ones that were doing a terrible job of it in the first place so then there is lots of low hanging fruit making the consultants look good.
Yet another item often missed: A/B testing success is a direct function of the length of the lever you are pulling. If that lever commands billions of dollars then it is easy to make it pay for itself. But if you're trying to turn $10000 into $11500 then you likely are wasting your time.