Underground Propane Tanks are just as safe if not more safe than above ground Propane Tanks.
Putting the Propane Tank Underground Protects it from things that might cause it to catch fire above ground and it gets the propane tank out of view which gives your yard a nicer look and provides more room.
Underground Propane tanks are made of steel but the propane tank installer will protect the underground propane tank from rust with a coating of something.
I can't remember what they used when they installed mine but the propane tank installer sprayed the underground propane tank with some rust prevention spray and then wrapped it in some rubber like stuff.
That's what my propane tank installer did but yours may do something else as it all depends on what they do to protect the propane tank.
As long as the propane tank that is installed underground was done correctly it should give you at least 40 years of service or maybe even more before it needs replaced.