I prefer the open-source equivalent https://responsively.app/ which is improving much faster due to community contributions.
Really at the end of the day I just want to see quickly what that change I just made looks across a few resolutions (mobile, common laptop and 1080p) without fumbling with window sizes, that's all. I don't need a million features that I will never use.
You can check out our recent updates here: http://fungarzione.com/sizzy
Tangent: as I've only recently learned about 'prefers-reduced-motion, I've become a bit obsessive about checking whether animation-heavy websites respect the request. Many websites, sadly, don't bother about it.