Slack and other ill-optimized Electron apps are not great, but Flash was far worse, at least in my experience. With Flash a single banner ad in some tab you forgot you had open could consume half or more of your G4/G5/C2D, whipping your fans into a frenzy and roasting your lap (if using a laptop). Slack, etc are a detriment in terms of heat and battery life too, but not to such an extreme extent and at least you're getting a reasonably functional app in exchange.