It's really about the aggregation of reviews. If everything is fine until 6 months ago, and then there is a steady periodic stream of negative reviews referencing a management change, then you can pretty well-assured there was a legitimate shift in the organization that made things worse. If it's just a trickle of some positive (HR lackeys posting fake reviews and/or current employees trying to reassure themselves about how they work at the coolest place in town) and some negative (often bitter people who've been laid off or dismissed trying to project blame onto the company), then you just have to look for the consistent elements and take what you can from them. No easy formula.
For startups specifically, I used to analyze the startups that VCs tweeted the most. Uber dominated for a long time. And Zenefits was tops too, until they dropped fast in mentions a few weeks before their turn of fortune. Yeah, some tea leaf reading and eye of newt stuff, but interesting nonetheless and some job seekers found it helpful.
Your point about departing/departed employees and hellscape made me think about how the recent negative news about Google contrasts with one indicator (again, imperfect) of how happy people are to be Nooglers and Googlers -- their public exultation in Instagram posts: https://gxjam.com/ig-gallery/noogler-instagrams-by-googlers-...
Edit: Of course, neither of the above approaches are as broadly applicable as Glassdoor's approach