Pretty much a ditto on what Tom says.
With an RP, you have to allow for the situation where one of the backflow seals has become compromised and water starts gushing out of the discharge hole in the RP. Under that worst case situation, a 5-gallon bucket is going to quickly be swamped.
About the only situation that I could see an RP being allowed to be burried would be a situation where it's on a hill such that there is an open drain to lower elevation so that the underground chamber can drain at a faster rate than the RP could discharge water.