That's not true. Servo is new, which is where Mozilla are picking their changes to Gecko/Quantum from. These false blanket statements aren't convincing me to use Vivaldi at all. Sure it has it's fair share of issues, but it "works" to a degree and maybe it'll chug along enough to finally be used by 2022.
Currently I'm using Ungoogled Chromium [1] on Windows and Bromite [2] on Android, but the latter will eventually be replaced with the former when I end up building Ungoogled Chromium TriChrome (it's a build mechanism for Android 10 and above to have a shared library package, as well as a WebView and browser package separately for Android).
Ungoogled Chromium is great because it accomplishes all that Vivaldi does, except syncing, all the while keeping the nice UI of Chrome. I'm not a fan of syncing, so that doesn't matter to me.
I used to use Firefox on desktop (prior to and after the Quantum update) but WebGPU was just way to slow for me and I switched to a Chromium-based application.
[1] - https://github.com/Eloston/ungoogled-chromium
[2] - https://www.bromite.org