Sorry, but both answers still fall short of the mark. When doing research, I seldom do it at one go. So, I cannot just select all notes and assign the tags all at once. In addition, I sometimes use previously tagged notes to create a first cut and review tags as needed. What is the difficulty about not bringing back the "hide the unassigned tags" option? Just adjust the previous code to the new Evernote.
I understand all the reasons that justified building a new app to make it an unified code across all platforms. It will be easier and cheaper to maintain. It also expedites features release in all devices. However, the new app still lacks many features that permit an user to have a more efficient experience and better control of the note format.
Hopefully, the development team will manage to "restore" some of the functions that make Evernote a quick and effective way to capture data from multiple sources in multiple devices. Right now, I prefer using both the Legacy and the new app on my desktop. The former gives better control on the notes formatting and faster productivity. The latter provides better compatibility between the Web and the desktop app.