I’ve had the RELION RB-100HP for almost 3 years now.

I use for cranking, (2) HDS Live + Livescope and it holds strong.

most of the guys on bbc boards are running ionic or impulse

Looks like Impulse does not make a starting battery after looking at their website.

If you have a motor that is still in warranty you better call your dealer and make sure it won’t effect your warranty.
There is not one outboard manufacturer that list a lithium battery in recommended starting batteries.
I know one person that was denied warranty on an electrical issue who has a Yamaha.
There isn’t a warning not to use them they just are not on the list of OK batteries to use.

the deep cycle batteries have the correct bms to run it as a cranking battery

I did a LOT of research on this topic when I bought mine last year right about this time. At that time the only Lithium I could find that had its BMS programmed specifically to be used as a cranking battery was the Ionic 125 Ah. I've had mine for a year and it's worked flawlessly. A good friend went with another brand and ran into the BMS shut down issue. It's very important the BMS supports cranking or you will end up dead in the water at some point.

what brand he have problems with?

I have one of those its a beast.

I believe they were Fuel Zero and to the credit of the owner of the company, he flat out told me his 100Ah battery was NOT rated for cranking and explained why. That was after I had placed an order which he refunded upon our discussion. That is when the research started and I ended up talking to Drew about the Ionic cranker.

Be careful with using Lithium on new motors. The fancy new alternators do not always think the same as the BMS on the battery....Could result in BMS shutting power off while under power.

The Relion 100AH HP starting/house battery has a charge voltage cutoff of 15.2 volts. Doubtful any outboard alternator is going to exceed that voltage output. I’ve been running a lithium crank for 3 years with absolutely no issues and incredible performance.