I'll start with their handling of COVID-19. They laid off around 100 or so people around April 2020, despite the business facing NO FINANCIAL HARDSHIP due to the pandemic. They went public two months later, for god's sake - they were fine. So they opted they put dozens of people out of work in the middle of a massive financial crisis for no reason? After we saw our coworkers disappear from Slack and were understandably nervous, they never fully explained the layoffs. It was addressed MONTHS later in a sentence or two in a Q&A email. I think it's disgusting that they used COVID-19 as an excuse to "trim the fat" when the business was actually thriving. It's just one example of ZI leadership caring about people more than profit (despite putting "we're successful because of our people!" all over social media).
I'll add one other point - there are a handful of people leading the company who are certifiably bad people. It's almost a joke among the employees - "oh, do not cross so-and-so, he will actually try to make you cry". Maybe you won't have to deal with these people in your own work, but do you want to work at a company that allows this behavior to continue?
HR clearly doesn't care about serious workplace complaints when it comes to certain individuals. It sends the message - it's fine for certain leaders to create a hostile work environment as long as they're making the business tons of money.
These are just a couple of problems out of many, but if I wrote about everything it would be a novel. Believe the negative reviews, and don't take the positive ones at face value. Many of these were left because an HR rep pressured them to leave positive reviews - I actually left a 5-star review because of this a while back.