Yeah... that number and % of negative reviews would be cause for concern.  Not a deal breaker on its own, but a reason for further consideration and investigation.  One thing you can do is ask the firm if there's a current employee in a similar role, with whom you can meet to talk about their experience with the firm.  Of course the firm will probably connect you with someone they think is currently happy, and that employee will probably be a little guarded with a candid opinion, since they currently work there - but you can at least get their feedback on the day to day life and organization of the firm.  

You do have to take Glassdoor reviews with a grain of salt.  As you said, those who take the time to write them are often those who have negative things to say.  Unfortunately those happy with their experiences are less likely to feel the need to comment.  Also Glassdoor is anonymous, so it's possible for one or two disgruntled people  to create multiple accounts and write a slew of negative reviews.

Another thing I'd do is try to get a sense of just how high the turnover really is, and the level of the roles that are turning over quickly (if it's entry-level people staying a year or two, that's pretty normal and not as concerning as if it's project managers constantly jumping ship.  It's also less concerning if the departures are staggered, than if people are leaving in large groups simultaneously, as that's often a sign of a firm that hires for large projects and then lays off between deadlines).  Some ways to do that is to look up cached versions of their site and see whether the staff list keeps changing, and to google their blog and media articles about new hires and such.  You can also look them up on Linkedin and check out the lengths of their former employees' stints there.

You can also bring it up point-blank, but tactfully, with the prospective employer that you came across these negative reviews on Glassdoor and are wondering what their take on that is - though if they're already aware of these reviews then they also probably already have a prepared spin on it.  

Some things, like benefits that aren't as good as other firms', can sometimes be negotiable, especially if you've got other current offers from other firms - so just because some former employees felt the benefits were bad doesn't always mean you can't get a better deal.  Other things, like a culture of long hours, aren't something you'll be able to change, so you'll need to evaluate whether that's a culture you want to join.

As someone who has worked in several offices in the past 12 years, I've found that consistent complaints on Glassdoor are typically on point. You can usually tell when it's just a disgruntled employee bashing the company, but if there is consistency among the negative reviews, it's for good reason.

Multiple negative reviews are a legitimate cause for concern, particularly if the same issue(s) are talked about in more than one review.

Also be wary if there is a mix of terrible reviews and ultra-positive ones.  I know of firms where efforts have been made to mitigate negative Glassdoor reviews by planting phony positive ones generated by firm leadership and the HR staff.  

All firms have problems, you would need to determine if the issues identified on Glassdoor for  firms you are interested in are something you could deal with or not.