It opens just fine on my Mac running High Sierra 10.13.4. Double-click the app and the drag ’n drop window opens. Drag your HEIC file onto the window and drop it. You’re given choices of JPG or PNG output, output quality slider and output folder. Click “convert” and a few seconds later, it’s done. You can click the button to go to the converted file or just quit. Couldn’t be simpler.
Where I think some reviewers may be having a problem is that when you close the drag ’n drop window it doesn’t come back on when you click the app icon in your dock. Instead, you have to go up to your menu at the top left corner of your screen, next to the Apple icon. You’ll see the “iMazing HEIC Converter” menu. Click File > New to open the drag ’n drop window, or File > Open to go to a folder and select the HEIC file to convert.
If all you want is a JPG or PNG file, this works just fine. However, if you want more conversion output options you can always double-click the HEIC file to open it in Preview, then File > Export > select a format (JPEG, JPEG2000, OpenEXR, PDF, PNG, TIFF) and Compression (None, LZW, JPEG, Packbits). There is no output quality selection.