People aren't understanding how much of a pain instant regeneration is in a fight.
Saturn is by far the biggest problem on the island until someone figures out how to shut down his powers. He has a long range attack that cause people to bleed from their heads, if not explode them, the ability to freeze peoples' motion, and instant regeneration of damage to his body, and enough base strength to pierce Kuma's body with an unnamed attack.
Kizaru's only advantage over Saturn is his speed, and, as we saw, Kizaru is not fast enough to avoid Gear 5 fully, which makes sense because Kaido had top-tier speed and Luffy could keep up with it in Gear 5.
Saturn's casual eye glare did more damage to Gear 5 Luffy than Kizaru did to Gear 5 Luffy their entire fight.