Picard is a special case: He was not turned into a regular drone, but into Locutus of Borg with the specific purpose to use Picard's expertise to lead the Borg's assimilation of the Federation. I don't recall this being explicitly stated on screen, but seems very natural to me that this meant that Picard's transformation into Locutus was less destructive to his personality than a typical Borg assimilation.
We see other cases of Borg drones becoming individuals again (Hugh in TNG, several examples in Voyager, most prominently Seven). However, they didn't return to being the person they were before. Seven is Seven, not Annika. Unimatrix Zero aside, it would seem to me that killing a Borg drone isn't killing a person - the person they were is gone, and the person they might become if liberated doesn't exist yet.
This is perspective is reinforced in the first season of Picard, where we see a derelict Borg cube being reclaimed. The liberated Borg drones are referred to as xBorg, and this seems to be their primary social identity. They don't just get some surgery and then go home to resume their former lives.
From a medical perspective it seems to make a big difference how long someone was a Borg drone. Picard was only Locutus for a short time, and it was possible to just remove the Borg implants and heal his body. Seven's borg implants are so deeply integrated into her body that the Doctor couldn't remove all of them. (Of course this isn't relevant to the recently assimilated crew members in First Contact.)
