Anyone who took the pro-police propaganda in G senjou no maou seriously would have a terribly warped view of the Japanese police, as the reality is far different:
Japan is peaceful, safe and regimented not because of, but despite, the frequently disgraceful performance of its guardians. Individually, many Japanese police officers are honest and dedicated. But as an institution, the force they serve is arrogant, complacent and incompetent.
Source:
Japan's Inept Guardians - NYTimes.com
http://www.nytimes.com/2012/06/25/opinion/japans-inept-guardians.html?_r=0
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As an aside, the gap between people's perception of the police and the reality is even bigger in the US:
The Real ‘CSI’: How America’s Patchwork System of Death Investigations Puts the Living at Risk
http://www.propublica.org/article/the-real-csi-americas-patchwork-system-of-death-investigation
CBS News Investigates Rape-Kit Backlog
https://www.rainn.org/news-room/news/rape-kit-backlog
Riding Along With the Cops in Murdertown, U.S.A. - NYTimes.com
www.nytimes.com/2011/04/17/magazine/mag-17YouRhere-t.html
There are also plenty of moments in which the police in G-senou no Maou are shown in a negative light.
For example:
I don't know about you, but I know for a fact that I wouldn't assume that the Japanese police are amazing and perfectly capable simply because a visual novel portrays them as such.