This is hard to argue for/against, because having one (native or cross-platform) generally precludes you from simultaneously having both (necessary for a proper comparison).
Of note: I find Slack's Electron client as good as, if not better than, many native applications that I also use in my workflow. There are also benefits I know exist (as a dev) that I don't see (as a user) like shared code that shouldn't be discounted just because the end-user can't see them -- they indirectly result in a better product for the end-user by making it easier to add new features and maintain existing code.
Also: I've never had any issues with Minecraft's cross-platform java client either. Would it be any better as separate native apps targeted at each platform?
That says nothing except the sad state of the other native apps you use in your workflow.
The truth is that yes, indeed, I can find a lot of native apps that are just about as "good" as slacks native Mac (and iOS - it's really bad too) app, and I can also find a lot that are a lot worse. But why compare to them at all? We should be measuring how good an app is by comparing it to, in this case, the best of the best native apps on the particular platform, or across all platforms, if so desired.
These things _used_ to work in the Slack app. What happened?