The following issues are already known and do not need to be reported again. (The same holds for all other bugs for which there are already open issues, but these bugs in particular get lots of duplicate reports, so they're listed here.)

• IME support (in particular various Asian keyboards) is broken on MacOS ((MacOS) Could not type with Asian(ja-jp romaji, zh-cn pinyin) IME #318)
• Problems with RTL languages and bidirectional text in the subtitle edit box (e.g. Problems with writing in Arabic in MacOS #331)
• Cursor drift in the subtitle edit box on Linux (DPI Scaling Issue on sway/wayland #432 and Wayland cursor offset #438. All instances of this should now be fixed on wxWidgets master - update your wxWidgets or see Wayland cursor offset #438 for other workarounds.)
• The screen color picker is broken on all platforms other than Windows (e.g take the color issue on Linux arch x11 #408)
• No dark theme on Windows (Dark mode #297). Check the linked issue for a detailed explanation of the current situation.

Note that many of these issues are upstream issues, i.e. not directly Aegisub's fault. This doesn't mean that they won't ever be fixed, but it can mean that fixing them may take longer than for Aegisub-specific bugs.