I would love for a trustworthy Glassdoor-like source, but I think there isn't a business plan that justifies the cost of building the network that doesn't also have the end goal being selling out to the employers.
I'll illustrate with a personal anecdote, recently I was looking at levels.fyi negotiation service, and I thought to myself $1800 seems way over priced! but then it dawned on me, so long the EROI is > $1800 in better offers, then I might as well use it, right?
For what workers lack in individual power, they overwhelm in aggregate value, if a service could provide positive value on $10 a month to a lot of workers they could make bank. Would you pay $10 a month to
1. Have a better manager
2. Get paid a high percentage of your true maximal value
3. Avoid companies that require therapy during/after the fact
4. Increase your inter-company mobility reducing periods of unemployment
An ideal glassdoor should be able to provide this.
As I write this out I kind of realize this is kind of like a market solution to unions.