In the past Vivaldi used their own user agent string and they ran into a bunch of issues. And they are a chrome derivative! They had to default to the chrome user agent. Here are the examples they cite in their announcement of the decision:

"On Google.com if you present a Vivaldi user agent and arrive via a redirect, the search text box will be misaligned

On Google Docs if you present a Vivaldi user agent you will receive a warning

On Facebook’s WhatsApp web interface if you present a Vivaldi user agent, you cannot enter the site and are advised to switch to one of our competitors

On Microsoft Teams (chat and collaboration website), presenting a Vivaldi user agent will stop you from being able to use the website

On Netflix, presenting a Vivaldi user agent results in a suggestion to install Silverlight to play videos… yes… really… Silverlight!"

(https://vivaldi.com/blog/user-agent-changes/)

A lot of systems seem to silently fail on Firefox - my broadband supplier's website failed at the last step of the onboarding process. I managed to get charged for installation (connection by the network operator, UK) twice, have onboarding emails sent, but not have the appointment in my account.

Used Edge, went through completely.

Can't guarantee it was Firefox/browser issues. But this is not that uncommon an occurrence.

I suspect it is the bank who are at fault.

Common enough that Mozilla has full-time engineers working on triaging compatibility issues, so they can either be fixed in Firefox or reported to webmasters. Here are the reports they get: https://webcompat.com/issues

I've never had one tell me; they just don't work, or they get stuck in a loop until they consume all my RAM and the gecko engine crashes. That is, assuming they even show me the page, instead of telling me to go away because they they think I'm a bot.

YouTube is crippled in Firefox, has been for years. It doesn’t force you to use Chrome, just a little nudge

Your experience may be different, but every time I hit the Cloudflare "checking if your connection is secure" turnstyle, it goes into an infinite loop on Firefox. It's the only reason I still have Chrome on any personal device. It may be tracker and privacy settings rather than just Firefox on its own, but I'm not going to run combinatorial experiments to figure out exactly what Cloudflare is looking for, especially since it's probably a moving target.

And honestly, I've only ever once encountered a website that required Widevine. And that site was a media site. So if you don't watch DRMed movies in your browser then you don't need Widevine in my experience.

I actually ran into such issues, in particular with commercial websits. Some browsers I use do not work for my online transactions for instance - annoyingly the local bank I use for logging into my account as well. It is basically the bank hijacking my money and forcing me into using a specific browser (or, at the least, very few; they improved compatibility a bit in the last years, but there were more issues in the past here). It is just a reality of the situation that some websites don't work well on certain browsers.