If you want statistics to back up either side of this, you only have to look at the US which does permit single-engine flight over cities and has many single-engine aircraft in public safety roles.
Honestly I have no idea which side of the argument those statistics would support. But anecdotally I don't see a huge number of accidents due to single engine helis making unfortunate forced landings in congested areas and taking out primary schools / kitten farms / whatever, or even the crew.
Again anecdotally, the main cause of heli fatalities in the US seems to be flying in inappropriate conditions. You could have eight engines driving four independent rotor systems and it wouldn't help with that.
But the statistics are there if anyone can be bothered to analyse them.