Since Yngve's article, I've been testing Vivaldi running without sending any Client Hints, and not noticed any site issues - at least on the sites I use daily.
I believe Client Hints is just Google once again forcing their own invented "standards" on the web, by reducing the UA in Chrome and forcing sites to adapt to Client HInts - which of course give much more information about the user's system than the UA string, hence improving fingerprinting and tracking.
Web developers (the stupid kind, of which there are many) will be able to fine-tune their responses to specific browsers and OS, effectively locking out anyone using minor-league browsers like Vivaldi: "We only support the most recent Chrome and Edge browsers".
Here's the JSON from ModHeader:
[{"headers":[{"enabled":true,"name":"sec-ch-ua","sendEmptyHeader":false,"value":""},{"enabled":true,"name":"sec-ch-ua-mobile","sendEmptyHeader":false,"value":""},{"enabled":true,"name":"sec-ch-ua-platform","sendEmptyHeader":false,"value":""},{"enabled":true,"name":"sec-ch-ua-arch","sendEmptyHeader":false,"value":""},{"enabled":true,"name":"sec-ch-ua-bitness","sendEmptyHeader":false,"value":""},{"enabled":true,"name":"sec-ch-ua-full-version-list","sendEmptyHeader":false,"value":""},{"enabled":true,"name":"sec-ch-ua-model","sendEmptyHeader":false,"value":""},{"enabled":true,"name":"sec-ch-ua-platform-version","sendEmptyHeader":false,"value":""},{"enabled":true,"name":"sec-ch-ua-wow64","sendEmptyHeader":false,"value":""}],"shortTitle":"1","title":"sec-ch-ua-OFF","version":2}]