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
This year's average is 50% of total rating
Last year's average is 30% of total rating
Previous year's average is 10% of total rating
All prior years' average is remaining 10% of total rating
Instead of years, it could be a rolling year, so average of last 52 weeks would be 50%, prior week 53-104 would be 30%, etc.I can offer one counterexample. I worked at a fairly awful place two and a half years ago and the Glassdoor reviews reflected that awfulness. At some point in the last year, management got wise and started planting hilariously, obviously fake positive reviews to go alongside the negative reviews. HR also started leaving comments on every single review, which had a chilling effect.
The average rating would certainly be higher over the past 18 months, but it's not because anything has improved at the company. They're just better at exploiting Glassdoor.
1. Are usually hilariously easy to spot. 2. Beget more angry reviews with 1 star rating in attempt to neutralize their effect.
Hence, I'd read them pretty carefully to look for patterns.
So at the very least, a wave of negative reviews is an indication to check the fundamentals, has this pivot worked, etc... Useful information.