I have no idea what people keep talking about when they say Arch/CachyOS is “unstable”.
a) stable in the sense of not changing, or more specific not changing dramatically the user experience and not breaking compatibility, so you can rely on being the same, and b) stable in the sense of not breaking by bugs and issues. Those are two different topics sharing the same wording of stable/unstable. It's a similar issue of the word "Free", which could mean either Open Source or no cost in money.
Archlinux is unstable, in the sense that it changes frequently, without warning for most of the time, and this can break compatibility or user experience. Compare the other extreme Debian, which is very stable.