Electron has its place, which is allowing web developers to make 'native' apps without learning new technology. When you are downloading any free software built on Electron, realize that it likely started as a labor of love that the person decided to open source or make available for free.
That said, large companies using Electron is what I don't get. You can't expect me to believe Slack doesn't have the resources to create an actual app for their product.
It's not that they don't have the resources, but if you want to create something that works the same on Mac, Windows and Linux it might even be a good decision for a bigger company.
Also they might have started in electron and now it's hard to move away from it.
That said, large companies using Electron is what I don't get. You can't expect me to believe Slack doesn't have the resources to create an actual app for their product.