Really want a way to have vi-mode input in all text inputs throughout the OS, hopefully that is a future possibility
In the fallback mode, not all motions are available, because we can't read cursor position or text field value without accessibility support.
some other useful ones are C-h/C-d (backwords/forwards delete), C-o (insert line), and the main Emacs movement keys C-n, C-p, C-b, C-f. Remapping Capslock to be a second Control makes using these regularly much more natural. Karabiner is a good app for easily remapping keys on macOS.