The problem has been around for long time and I no idea why it not been addressed…
Engineer Combat drone are just not suitable for well… combat.
1 explosive, and lets face it explosives are everywhere, and the combat drones are destroyed, you then have long wait for them to respawn, but by then you been hit by another explosive.
Combat drones basically just get stuck in a constant respawn loop, and they only of any use when out of combat, even then they only fix upto 50% shield.
Only in very rare occassion when your enemies are out missiles are the drones remotely useful.
In addition to the above, they attack above 50% shield, and heal below 50% shield is also pretty poor, because it takes seconds for the shield to get below 50% so the drones dont really have chance to fire, drones are generally only useful in PvE scenarios.
1: Up the health of the drones so they can take at least 1 missile hit, and allow them to regen health if nanocloud is active. a second missile or explosive will still take them out and most prob before they can heal.
2: Change the combat behavior to proximity based, rather than shield/health based… eg… if they hostiles near they will attack, if they no hostiles in range they will repair
After all its pointless trying to heal the shield when your still taking damage, coz the damage will be far more than you can heal… least by attacking them you might make them fall back for a brief moment.