It has to be noted that they do not heal Toxins at all.
A working Surgical Unit can make many areas of the game much easier. As noted above, most of them are incomplete, so you'll need Activation Keys to give them full functionality. Badly damaged (blood-splattered) Units are inoperative and cannot be activated nor used.
Since Activation Keys are fairly rare (only seven of them in the whole game) and they cannot be removed once installed, you should think carefully before installing one. Is this an area where you think you're going to need frequent healing? How near is the nearest working Surgical Unit behind you?