In terms of physical damage to brain, or by that I mean rearranging how your brain interacts with drugs and other "naturally" produced chemicals, from my own experience opiates are some of the most forgiving drugs in the world.
I mean, I've surely been severely addicted to a number of them, from hydrocodone (to the point where I was dumb enough to take 12 vicodins a day for two or three weeks - with all that apap included!) to heroin (up to a 3 bundle a day habit - intranasal) to suboxone to pods (worst withdrawal experience ever imho) and pseudo opiates like kratom and tramadol (so much more forgiving with habitual use, although not to be underestimated in any way).
These drugs have had a much MUCH less negative effect on my life than my previous cocaine, methamphetatmine, amphetamine, mdma, benzo/z-drugs use. In fact, the only ways I really have see opiates affecting my life (basically going on a four to five year addiction now) in a "structural," interpersonal relationship sort of ways way, if that makes sense at least. They basically, and I'm not saying for the better but not necessarily for worse, affect how I go about my "normal" routine. They affect how I deal with social interaction, school, work, etc.
Often, they've had a horrible effect on what I do and how I treat people - i.e. at one point they led me to cheat, steal and lie on a regular basis, even to those I most loved and continue to love, back in my early heroin days. But now after learning how to control my use and after having to deal with the (horrible, yes) consequences of my actions related to opiate use, they have had a much less negative impact on my life - in fact, given some long time intense depression problems, they are more and more affecting me in positive ways. That is, since I've learned how to "control" my use (at least to a reasonable degree - i.e. not cheating stealing and lying).
But in terms of major damage done to the brain, seratonin levels and such, well, I haven't had much of a problem with them, at least in such negative ways as one might worry. And at least, that is, since I've learned how to control my use. In comparison to my past cocaine use, that REALLY messed me up in the head, like horrible depression resulting from prolonged excessive use. I've had a similar effect from irresponsible mdma and RC use.
However, from my experience with almost every drug I've encountered (23 years of age) now, I find that the most dangerous drugs in terms of changing the way your brain functions, you have much more to worry about in more long term ways is more with drugs such as benzos, z-drugs and amphetamine/methamphetamine drug use, especially if it's irresponsible, habitual and, well, shall we say more than used on a regular basis. Basically, I'm much more concerned with my use and overuse of things like xanax and amphetamine.
Compared to drugs like those, I find opiates to be REALLY benign on my body and how it works (again, there is the caviate of addiction, but that's a whole other bag oh worms, as others have mentioned). Opiates, short of the problems related to moderate-severe addiction, are a pretty forgiving type of drug, at least on its impact on you biology/neurology.
Hope I haven't been repeating myself too much here. Little xanax relaxation at the moment.