If the medbay is full and additional crew need urgently healing, the room can be over-filled by walking additional crew back and forth across the room.
The medbay only heals friendly crew, so fighting boarders inside your medbay is advantageous. Boarders can be herded into it by selectively venting other areas, as they flee rooms without oxygen. If your crew are mind-controlled in the medbay, it will not heal them; likewise, your medbay will heal mind-controlled enemies.
A level-2 hacked medbay can kill all crew trapped inside (except full-health rockmen?). Trapped enemies will immediately try to escape a hacked medbay, so ensure the door isn't broken.
Having an improved or advanced medbay adds a blue option to several random events. Most of these events are linked to disease and allow the survival of your crew or rescued people.
Healing boost: 1x
The healing rate is equal to the suffocation rate, 6.4 health per second.
Healing boost: 1.5x
Crew can heal in an airless medbay.
Healing boost: 3x
Strong enough for one crew member to fight off several boarders.