بسم الله الرحمن الرحیم
Hi.
They are not completely the same. RBE is a rigid beam element (infinite stiffness) but when u use an explicit constrain it is like separating a plate into tow pieces, e.g. when u connect tow parts of a beam with explicit MPC the slope of the beam before and after the mpc remain the same but RBE2 acts like a part of beam.
use qrg (quick reference guide) of NASTRAN for more.
Regards.
Ehsan Badri Kouhi