In fact, it’s all too powerful for some simple functions. Like holding a key for a modifier instead of the normal character, is entirely overkill to implement with a Tap Dance. Especially when that basic function is already there in QMK/Vial in a much simpler format called Mod Tap.
Keymap -> Quantum tab you can find a long list of keycodes that create the exact scenario above, where you have a modifier on hold, and then select your desired ‘normal’ key-code for tapping.
Please read more in QMK Documentation : Mod Tap