Even a perfect reentry would subject the Apollo 8 command module to extreme stress. With Gemini, the capsule re-entered from Earth orbit, but Apollo 8 would re-enter at approximated 25,000 miles per hour. The forces of heat and deceleration would be much greater.
Kraft, Gilruth, and Trimble in Mission Control
Carr, Slayton, Armstrong (seated), Schmitt & Aldrin (standing) compare Lunar Orbiter photos with Apollo 8 TV pics