Glassdoor seems ripe for astroturfing. At my last company, HR specifically targeted new hires (they didn't want people that had already been there and been soured by the culture) in a mass email to please leave a positive review on Glassdoor. It's also just painfully obvious from looking at the reviews which ones are fake, and it's a lot of them.

This certainly isn't true for all companies, but Glassdoor isn't exactly immune to shilling.

Companies game Glassdoor feedback all the time. 50 negative reviews will get quickly drowned out when HR offers rewards to employees for filling out a Glassdoor review.

How reliable is that data?

I'm kinda skeptical of its ability to provide any real insight company to company.