It is absolutely mind boggling to me that Evernote does not include operator grouping. The idea that adding this would require onerous rework to their search engine only means that whoever designed their search engine should be fired for incompetence and sued to reclaim the wages they were paid. Google search has offered boolean searches since the dawn of time, and it is impossible that the Evernote search architect had never used this feature in Google. If they didn't want to implement it at the onset of the coding, that makes sense...but there is no excuse for not including the capability in the engine.
It is also extraordinarily unlikely that this would confuse users. If a user can master "todo:false -tag:Meeting" they can master "todo:false (tag:Meeting OR tag:Seriously?)".