While I understand that people hardly take the effort to write positive reviews even if they've had a positive experience - this doesn't mean that all the negative reviews are genuine either.
I've personally seen many disgruntled employees who have posted absolutely false and misleading reviews created with pure imagination for malicious intent.
I'd say, the best way is to make reference calls with random folks from that company (not just 1 but at least 2-3) to get a first hand review. You can even take ex employee reviews.
And remember - there is not a single company out there that's perfect. There's a lot of variables and moving parts in any organisation - this includes our own (employee's) imperfections too.