ST_SphericalBufferMBR and ST_SpheroidalBufferMBR basically perform the same function, but with different performance and accuracy. ST_SphericalBufferMBR models the earth as a simple sphere, so it performs better than ST_SpheroidalBufferMBR, but with less accuracy. ST_SpheroidalBufferMBR models the earth as a spheroid, so it returns a more accurate MBR, but it runs slower than ST_SphericalBufferMBR.
The returned MBR cannot cross the longitude value of +/-180 or the latitude value of +/-90. The MBR cannot cover more than 180 degrees of latitude or longitude on the sphere or spheroid.