What are the risks in writing a bad review of ex-employer on Glassdoor.
In theory there are none, because the posting is done anonymously*.
It could be a bit evident if the company has few or almost none reviews, when that company could draw conclusions and wonder on who might have been... but this is still conjecture.
On the other hand, by writing a truthfultruthful review you may spare future workers a bad experience.
Some related reading (a post from Glasdoor themselves): Can My Boss or Employer Fire Me For Posting on Glassdoor?. Also may read Tips on writing a review to avoid defamation.
* It seems that according to this article from last year, the Federal Court decided that post may not be too anonymous after all, as the government could compel Glassdoor to provide user info for investigations.