There’s various downsides to ChromeOS including how it handles encryption and issues with proprietary systems in general.
I don’t know much about ChromeOS in particular, I’ve heard rumours that it’s comparable to (or worse than) Windows 11. Based on the company’s track record it’d be fair to assume it’s probably bad.
Probably due to privacy concerns. Privacy Guides also prefers open source solutions which ChromeOS is not.
The “Linux insecurity” fear mongering is way overblown, Secureblue or even Fedora Workstation should be just fine for your use case. And as mentioned previously, there are various security issues inherent to proprietary systems like Windows, macOS, and ChromeOS. Whichever is “more secure” isn’t clear but rather more dependant on your threat model.
Generally speaking QubesOS, macOS, and ChromeOS are all bad options for PC gaming. Windows obviously has the best compatibility but Linux comes in a close second. The out of the box gaming experience on Secureblue probably won’t be great. Fedora Workstation might be a bit easier but if you don’t have a reason to obsess over security, you might even consider a gaming-oriented distribution like Bazzite.