I prefer "uncapable of reentry". FAST pack are literally missile ammo with extra fuels, the massive heat during re-entry might trigger an internal ammo explosion. Even in Macross 7, the VF need some re-entry vehicle (Operation Stargazer) so some of the surviving valk can reach atmosphere safely with their FAST packs.
However, Sound Force are capable to perform re-entry with their sound booster (Galaxy is calling me) this might be explained as their sound booster doesn't contain any fuel or missiles, the sound booster are just huge speaker with holographic system.
PS: YF-19 and YF-21 is equipped with augmentation pack, not a real FAST pack.