I'd personally think "amazing" is a better adjective than "unfortunate" here - the value of an e-mail client, at least to me, lies almost entirely in its user interface. If using a native stack can squeeze out a few more tidbits of usability here and there, that's a good job well done. There's still a lot of value in native if it's done properly.
Electron and similar technologies are in my opinion not quite at the point where they can compete with the best that native platforms can offer. I'm sure that will change one day, but we're not there yet.
GTK integration on windows and mac is also less than perfect - I've been using geany on multiple platforms a lot recently and it's a great tool, but there's small annoyances like that the "swipe to scroll" speed varies massively between it and Notepad++.
Electron and similar technologies are in my opinion not quite at the point where they can compete with the best that native platforms can offer. I'm sure that will change one day, but we're not there yet.
GTK integration on windows and mac is also less than perfect - I've been using geany on multiple platforms a lot recently and it's a great tool, but there's small annoyances like that the "swipe to scroll" speed varies massively between it and Notepad++.