Using dot or underscore to generate tag hierarchies is great. They have the amazing side-effect of creating outlines in the Tag pane. However, they don’t share the same function as a simple tag.
When I click on a simple tag ( a single word without separators ) it selects that tag in the Tags pane and reveals the list of notes. Clicking on any tag that does include separators will not afford the same pleasure. The most it will do is collapse any currently open tag lists.