They both achieve the same thing in different ways, but it comes down to philosophy if that even matters to the user. You can build gentoo for example with openrc or systemd, and depending on what you do you'll need to integrate things differently (using elogind with openrc since logind is systemd specific). In gentoo it affects how you'll compile. Its really a non issue, but it all depends on what level folks wanna spend on something that could potentially be different. I use openrc daily on multiple setups and it took maybe a week of using it to get the hang of it. Its just operationally different.

Does it really matter for most folks? No. But if its something you find interesting, it can be a nice change based on personal views and principles. I run openrc because I personally as a programmer don't believe in how "proprietary" systemd is designed, nor agree with the decisions the maintainers have made. That's just my opinion. At the end of the day it doesn't technically matter.

This is a more hardcore viewpoint but it covers a lot of the issues folks have with it. https://suckless.org/sucks/systemd/

I use Gentoo which originated openrc but also enables systemd.

They're both fine. Both will do the job. Systemd just can do more without additional packages or can be configured / built to be pretty minimal.

i used them all, i didn't notice much difference. but openrc has the coolest bootup screen by far

At the end of the day, they all achieve exactly the same experience. The only real difference is that systemd does a lot of the work for you. Personally I wasn't a huge fan of micromanaging my services manually on openrc or s6

Not sure what firmware limiters you're talking about? I'm using a cheap ASUS board (B450i-gaming), a Zen 2 CPU and a 6600 XT, and single GPU passthru works just fine for me on Arch using this guide. (I haven't tried VMetal or this new release of Nitrux yet). Yes, some manufactures are iffy about IOMMU support, mostly Intel-CPU and Intel-based boards in my experience, but if you're using AMD you should be fine.

There is something called an ACS override patch, but that's a kernel patch not a GPU firmware patch, and from my understanding, that's for dual-GPU users. Regardless, it doesn't modify your firmware in any way.

Immutable in this context usually means the root filesystem is readonly at runtime, and all changes are performed by updating a set of declarative config files that describe the desired state of the system. Changes would be prepared and applied after a reboot. Something like that.