You rarely want to change every file the same way. For example: "If Modality equals 'CR' (Computed Radiography), then add 'XRAY' to Study Description. If Modality equals 'MR', do nothing." Advanced batch editors allow regex (Regular Expression) searching to find partial string matches.
While primarily a viewer, Weasis includes a robust DICOMizer tool that supports batch modification of tags via scripting. It is slower on very large datasets but free and open-source. quick dicom batch editor
The best batch editors eliminate the "File -> Open" dialog tree. You should be able to drag a folder hierarchy directly into the interface. The software must recursively scan subdirectories instantly, presenting a flat list or a tree view of all series and studies. You rarely want to change every file the same way