The problem is on-boarding - once the user learns to ask for computer a question then the text (or voice) interface has the potential to be on a completely different level than a graphical interface.
Just imagine - instead of searching through a dozen menus for an option you just ask.
Hell, that's how most people work with Microsoft Office now - fumble around trying to find out where they've hidden the option. Fail. Open Google and type "how to change signatures in Outlook 20XX".
Slack solve that brilliantly - their on-boarding is well designed - the way it gradually introduces features and reinforces concepts is excellent. It reminds me of modern videogames in design.