is it able to exile even uncounterable spells?
When a rule, spell or ability refers to a keyword action (a verb with a rule definition), it strictly refers to that action.
Pearl Lake Ancient can't be countered (a keyword action), but it can be exiled.
Difference between countering spells and exiling spells
To counter a spell, move it to its owner's graveyard.
"Exile" isn't a keyword action. It's simply short for "move to the exile zone".
In both cases, the spell will never get a chance to resolve since it will never be the topmost item on the stack after all players have passed in succession.
- "Move" also isn't a keyword action. That's why many different verbs are used for the action. The verbs include: "move", "put", "place", "return", "exile" and "die".
- Keyword Actions
701.1. Most actions described in a card’s rules text use the standard English definitions of the verbs within, but some specialized verbs are used whose meanings may not be clear. These “keywords” are game terms; sometimes reminder text summarizes their meanings.
701.5a To counter a spell or ability means to cancel it, removing it from the stack. It doesn’t resolve and none of its effects occur. A countered spell is put into its owner’s graveyard.
608.1. Each time all players pass in succession, the spell or ability on top of the stack resolves. (See rule 609, “Effects.”)