All HP items are bad on Rhaast
I always find far more success with lethality rhaast than bruiser Rhaast. I have absolutely no clue why people still build HP on him though. Black Cleaver takes too long to stack and deals less dmg compared to Serylda. Shojin is the only other "good" HP item you can build but pales in comparison to alternatives with no hp. There's not a single HP item that is best in slot on the champion. Stop building HP items on him and you will find yourselves winning more games than usual. It will take some time to get used to the lethality red kayn playstyle but I promise you that it's worth it once you get the hang of it. It also allows you to 1 v 1 champions like yone which otherwise you'd never beat in a 1 v 1.
The veasher has a good Rhaast page. I sometimes go his build but I mostly go lethality. I believe Bastion Breaker is also an incredibly broken item on Rhaast. Axiom Arc is an Item that I do not recommend though. I'd suggest getting endless hunger in place of axiom arc 100% of the time. DD and Endless Hunger are the only good bruiser items and neither of them gives you HP. A good build would consist of full lethality with DD and Endless Hunger. You always wanna go either tabis or omnivamp boots. Never mercs and ionian boots !!!
I can't think of a single HP item besides Shojin that is actually good on Rhaast. I hope all of you start running a no HP Rhaast setup in your games so that Rhaast's WR can increase and make the balance team wonder what's happening (again). They will likely give his Q hp scalings too so that we can actually build items like sundered sky and steraks without giga trolling our games. For now all HP items are basically just bad on Rhaast and will not change in a long time!