Not to be biased, but personal 9 years of experience tell me that if a program has a landing page "oh yeah, and for linux and mac too", it looks shit on mac.
That was an exaggeration, but many times programs that look identical (aside from window frames) across platforms don't show the window frames for every platform. Electron apps (for as unpopular as they may be) are pretty platform agnostic. If they show it on windows and just have a linux/mac download button its not a big deal.
skype.com probably fits the bill of looks shit, but it shows android and windows 10 screenshots only.
obsproject.com shows a lot of screenshots that look pretty windows 10-ish... but it also announces "Latest Releases <platform logos>" right at the top of the page.
sublimetext.com shows windows only screenshots. And then mentions platforms at the bottom of the page.
https://slack.com/is shows only mac screenshots. I imagine this reflects more on the developers than the actual product. Im pretty sure its available on other platforms.
And my point was less about quality of programs, but availability. showing the window frame of a single OS does not mean that only that OS is available, sometimes that's just the only OS the marketing team uses.
Perhaps others don't share this view but I don't care so much how it looks if it gets the job done, especially if there is no other alternative.