Why wouldn't the SOH be able to beat the Unsullied in a sneak attack on their home court?

I have to imagine that the SOH are made up of former Mereen police and military, as I'm sure Mereen had such institutions that were established by the rich, for the rich so its not just rich kids.

Besides that, it wasn't like the entire Unsullied was at the coliseum for the fights.

Mereen is basically GoTs Iraq.