Overall I was treated well at Dentsu. While it took a while to go from permalance to full time, ultimately promises were kept, even after original managers moved on.
The biggest problem with Dentsu is that there is a huge "stay in your lane" mentality. Mid management gets visibly annoyed when suggestions are expressed outside of the purview of your job description. Strong-willed and senior employees won't be dissuaded, however, it's very demoralizing to younger employees.
Compensation is on the lower side of the industry standard. Employee perks such as cell phone reimbursement have been slashed multiple times to the point where they'd be better off including it in salary rather than calling attention to how little they reimburse.
Politics play a role everywhere, but at Dentsu it doesn't seem as bad as other agencies.
Summary - Decent place to work, but I wouldn't plan on making your entire career there.