one of the key concerns was the impact on felix when going through the sound barrier (sonic boom or is that just phrasing rather than actual effect?) could cause him to spin out of control.
All sorts of nasty aerodynamic stuff happens at transonic speeds…hence it was such a big deal figuring out how to design aircraft to handle it.
OTOH he’s up in such thin air that the shock forces etc will be correspondingly reduced, presumably its not going to be anywhere near as bad as going supersonic at sea level.
The sonic boom is just the compressed shock wave you’re pushing out around you, which can’t escape out in front fast enough….the aerodynamics get weirdest when you’re just below/above the speed of sound so you’re stuck right in the shock wave with airflow that’s varying from subsonic to supersonic in different areas around the airframe (…or indeed, body).