It's still a 150MB web-browser + attached web app, right?
If you want a lightweight version of the slack client and don't fell like using the IRC or XMPP bridges you can use Epiphany (or Chromium) to install the website an an 'App'.
There is also https://github.com/bkanber/Slackadaisical (no development since initial October appearance) CLI and:
https://news.ycombinator.com/item?id=15597508 >nikisweeting: https://github.com/saenzramiro/rambox [...] https://github.com/meetfranz/franz [...] share one Electron instance for all your chat services
Pinning a tab gives it permanent priority on the left-hand side of the tab list, and elides the title from the tab, so it is just the favicon. It also blocks CTRL-w from closing the tab; you have to actually right-click and either unpin or close the tab. That may sound trivial, but was the biggest problem I had running it in the browser before, it got caught in my periodic tab cleanups accidentally. Now I never accidentally close it.
But either way its an option that probably wouldn't exist otherwise.
My real beef with electron apps is that they invariably lack support for all sorts of goodies (accented-character menus, OS-specific shortcuts, text auto-expansion, etc etc...) that regular desktop apps just get as standard.