1. Try to dragndrop content from a child folder to the parent folder. There is a very VERY small area at the top of the window where you can drop your files. (got worse on 10.13)
2. Transfer of multiple files (multiple actions) is not managed successivly but simultaniously.
3. Constant loss of sidebar content, i.e. mounted drives.
4. Info about folder/s inconsistency: Open info for one folder 'Cmd+i', for multiple folders 'Cmd+Alt+i' if you want the overall size (and it doesnt' get highlighted) | Close info window for one folder 'Cmd+w', for multiple folders 'Cmd+Alt+i'.
5. No option to edit file/folder names in Save As... dialog
6. Save As... dialog again: click on a file by accident and your initial file name is gone.
7. I don't understand how people can use the columns sorting but that's personal taste I guess.
8. Missing native window snapping (half size, full size etc.).
MBP 2009 OS X 10.11 & Hackintosh macOS 10.13.4 and my background is years and years of M$.
There are things I love about the Finder, too. :)
There's nothing really bad about Finder. It's just sort of meh for a tool that one uses all the time.
This tends to show up if you're trying to tweak/customize preferences (e.g. from the command line or editing plists directly), since you usually have to duplicate the effort for both places.
Even to call it a "system" dialog is misleading, since it's just a system-provided library that each individual app is, technically, free to use or not use.
I certainly agree that it's annoying when the look, feel, and fucntion of all the apps' common interactions don't match each other. I do recall a time, though, when the system library features were so anemic, that I was actually glad of app's departure from the norm if it provided a useful new feature (e.g. "New Folder" button in a "Save" dialog).
I also agree that makes the suggestion of "use a Finder replacement app" not a solution to your real problem.
Maybe I'm suggesting that, to a nerdy-enough audience, you might have to state your real problem as being with "the Finder and all the system provided file-picking dialogs other apps tend to use".