A nominal lunar return velocity is about 11.2 km/sec. LEO return is about 7.5 km/sec. The driving parameter is kinetic energy which scales as velocity squared, i.e. 11.2 * 11.2 = 125.44 versus 7.5 * 7.5 = 56.25 . The kinetic energy for lunar return is almost twice as much as LEO return. Also lunar return is dominated by radiative heat flux. For example the peak convective heat flux for Apollo 4 (AS-501) was 186 W/cm^2 but the peak radiative heat flux was 333 W/cm^2, Radiative heat flux (radiant heat from the shock layer) is much more difficult to design a heat shield against. The Apollo engineers were not confident of their design methods so they test flew the Apollo CM multiple times unmanned before they flew it with people. Mistakes were made with the Apollo CM (the predicted moment coefficient was way off). However as we all know, the CM worked beautifully in the end.

Gary