And if your OS breaks you can always boot from another one and recover your cluster.

Now try that with an obscure Adaptec card which has been EOL for the last 4 years :-)

And the SATA/SAS controller of your Mainboard is invincible? Or the Mainboard itself? Or the CPU? What's your backup solution if they are faulty?

I use two different controllers in JBOD mode. Each controller manages half of HDDs. Then I build cross controller software RAID 1. That is, I choose two disks from separate controllers to build raid 1. Last but not least, all HDDs are mixed from different vendors across controllers.

For CPU, RAM there is no other option. you have to replicate stuff.