There have been several characters in the EU with allergies to bacta - Ton Phanan comes to mind. He had a bunch of cybernetic body parts because he was unable to be treated by typical methods. I think there was also at least one character who died because they were stuck in a bacta tank by people who didn't realise they had an allergy?

On the sociopolitical side of things, there was a lot of interesting stuff around bacta as a limited resource in The Bacta War, but it's been ages since I read the X-Wing books so I only remember the broad strokes...