It seems that the idea of a Lithium battery as a cranking battery is split 50/50.
I have a Mercury Pro XS 250hp 4 stroke.
Why would I not want to have a Lithium cranking battery? Has anyone had issues with the motor due to the Lithium battery?

I was told Mercury would void warranty.

Mercury's parent company just bought relion.

https://boatingindustry.com/news/2021/09/02/brunswick-acquires-relion-battery-llc/

And to piggy back. Rangers come standard with a Tracker lithium and Nitro has it as an available option. Both boats come with Mercury motors, but the Tracker lithium is also made by ReLion. They were doing this before Brunswick actually purchased ReLion too.

Yup, its in the manual. They also said no synthetic oil but just released their own line. Now that they bought ReLion I'm sure it won't be long before they approve their battery.

I hope they do, I was told it was something to do with the engine alternator, I would suspect they will make changes to the alternator or the battery so they can sell them for cranking.

Lithium's have lower resistance. They charge faster because they keep that lower resistance and lower losses for a long time. Whenever you try to charge a lead acid battery it won't take as high of a charge rate and as it heats up resistance rises even more.
A lithium which has lower resistance will take a higher charge rate and stay charging at that higher rate over a longer period. Mercury alternators have two bad characteristics that make it hard on it.
They do not cool the alternator very well at low rpm's and they aren't built to sustain a high rate of charge over time.
Both contribute to the problem.

It can be solved easily enough with a heat sensor and circuit that would change the charge rate. Maybe Mercury has stepped up on done this.