Brass fittings can be used with galvanized steel fittings and galvanized pipe.
However the galvanized steel pipe and galvanized steel fittings can rust when it reacts with the brass fittings and be hard to remove and eventually leak.
But it's okay to use brass fittings with galvanized steel pipe fittings when needed.
Brass water pipe fittings can be buried and they commonly are buried.
For example brass water fittings are commonly buried with frost free yard hydrants.
The bottom fitting where the water drains from a frost free hydrant and where the water flows through it is brass and it is buried.
Other brass fittings are buried as well and can be buried and will last for several years.
Brass fittings when connected to galvanized steel fittings can cause the galvanized steel fittings to rust faster than they would if the galvanized steel was just connected to other galvanized steel fittings.
Even galvanized pipe and galvanized steel water fittings can be buried but they will eventually rust.
Copper fittings and copper pipe and tubing can also be buried.
Copper tubing is commonly buried as water main piping from the street to the house in NYC.