There are plusses to modern comments. For example, I love dark mode comments.
Some minuses that are getting under my skin:
Hyperlink Hashtag Bookmark - e.g. c:\mydoc.docx#Chapter_2 does not work. The hashtag does indeed appear in the full hyperlink in in the Modern Comment but clicking it only opens the other file and it does not go the bookmark. Excel honors hashtag hyperlinks, Powerpoint honors hashtag hyperlinks, Word Original Comments honors hashtag hyperlinks (as does LibreOffice etc), but Modern Comments do not honor hashtag hyperlinks. Of course, Internal links to Bookmarks (i.e. Insert Hyperlink, Place in Document) do not show up as a hyperlink at all in Modern Comments and you just get "Missing Content". Hashtag hyperlinks are critical for researchers and writers and save a LOT of clicking and scrolling time when files are large.
Rich Formatting - Feels like I am in dummy-Google/impoverished-texting land. Bold and Italic does not cut it. See Keyboard Shortcuts.
Keyboard Shortcuts - Modern Comments should honor all keyboard shortcuts that work in Original Comments like Tab, Shift F3, Ctrl Shift F, Ctrl Shift L, Ctrl Alt 2, Reveal Codes, etc.
Missing content - as in "This comment contains content that's not yet supported." I have a lot of those.
Editing - This should not bother me/us but it really does : I have to click a button to edit my own note. I want it to be like the old way where I just click right into the comment. It should know that i am the owner and take me straight in to edit where I have clicked in the text just like it knows that I have clicked inside the Reply box.