Why can't Kakashi use Lightning Cutter properly after losing his Sharingan? Is Kakashi's new technique Lightning Release: Purple Electricity as powerful as his signature technique, the Lightning Cutter?

On Naruto Wiki, it is written that after losing his Sharingan, Kakashi can't use Lightning Cutter properly. What does this mean?
Kakashi can't use the Lightning Cutter because it creates tunnel vision without the Sharingan, leaving him open for counter-attacks.