Glassdoor is an interesting data point, but not "reliable" as a standalone rating system for multiple reasons:

1. It tends to attract complaints and unhappy (ex) employees more than anything else. The ratings tend to skew low as a result. Look for common themes, and take them with a grain of salt.

2. Complaints can be specific to a department or role. Complaints about Amazon, e.g., may be related to working in one of their distribution centers, not in HQ/IT.

3. Current employees don't tend to leave reviews. Unlike an annual employee survey—which would be a better indicator if companies chose to publish them—Glassdoor tends to ignore current employees.