I agree. Hide works instantly, dash should too.
I can see confirmation being useful for abilities that use a ressource, [...]
Many things are resources. Action and Bonus Action are, and you get only one of each per round, which you can only use on your turn.
As you pointed out, Hide costs a Bonus Action, and there is no confirmation required. (Not that it should be so btw. Hide should cost an Action for everyone but Rogues, but this is outside the scope of this discussion.)
Dash costs an Action, and requires confirmation.
Personally, I fail to see why there is a difference. And I would welcome a more consistent UI. (Also, perhaps some UI optionality. Some players might want to have the game require confirmation for both actions —and plenty of other things. Some players might want to have to confirm their choice for neither action. Some players might even want to mix-and-match.)