Whats the deal with grounding? Why is it so confusing on which mobility laden abilities it prevents?

I'm finding the grounding effect doesn't work on things that rooted would normally prevent like Q/E, ult, Q-E dash etc.

I understand grounded is it's own thing but the tooltip says "A unit that is grounded is unable to activate mobility spells."

Now that the debuff is being spread around to champs like , it's a good opportunity to streamline its interactions and solidly prevent champs from using abilities of any kind to move while under its effects.