By real autocompletion dropdowns, I mean dropdowns which are different from editing buffer. That means whatever is the colour of the code/background underneath, they won't blend in in confusing ways. VIM does coloured completion drop-downs, but they're still just buffer-like text. Unless you mess with your theme manually, there's a chance it will blend in and confuse you one day, because there's no real frame around it.

As for markers, I know they're supported by Vim. That's why I'm saying better. For example Vim can't do single-pixel column marker (or specifically, create a line between columns). I can do hacks to highlight one column only (100th for example), but then highlight/copy on the terminal is broken, because in reality I just have spaces displayed after my code. Similar thing with region folds - they don't need to take the whole text row and the description of the fold can go off-buffer on a side.

Basically you can do everything with text. You can even print out "this is a placeholder for an image of a sad green frog looking at a computer screen". But actually displaying that picture is better ;)