Your point about the small companies is valid, and it used to be better. When Glassdoor was at its peak, you could find smaller companies more frequently and I would read up on the companies I did business with to get an employee's perspective on whether they were functioning effectively. If your employees hate your guts or think their job is pointless, that's also a bit of a red flag for me as a consumer. This Glassdoor research worked well for renovation contractors, larger service providers like electrical or plumbing, commercial real estate management companies. Sometime you could also find info that made it easier to navigate call centers designed to frustrate you into giving up. It looks like someone posted a few alternatives and glass door stopped being useful for company research almost as dramatically as google became ineffective for other research. Someday soon, all we'll have is the company's marketing slop and any honest opinion will be buried and hidden into non-existance.

With regard to review manipulation, I knew a company with an abysmal rating, a real w2 farm. The people I knew spanned entry level to the c-suite. Said company would have bootlickers in HR and elsewhere post 5 star reviews to try to move the needle. They also asked people to rate them well after training had completed and everyone still had "new-job glasses" on. Despite their efforts, I think they were still sitting at a measly 2.7 stars, which is still way higher than the 1.5 they deserved. The 0.5 is mostly based on the bottomless supply of decent coffee in the break room :D

I don't have interactions with many people from this company anymore, but what I have heard is basically "different people; same shit".