I can see how the junk turret is out of place. That is some high tech stuff going on. It is especially out of place when comparing to food poisoning. Under that same premise, a lot of things are of out place including the regular turrets. I'm not going to bother listing all of them because the list would be too long and frankly not worth the time. I'm also not going to bother getting into the whole realism thing with this game. There are more examples of non-realism than realism and that is completely acceptable for this game. This game does not try to be a simulator... that is some other game. However, some realism still exists. Not all physics are thrown out the window. The only thing I really agree with to conform to the game reality presented by this game is that they should require a battery. Gunpowder is unnecessary. The battery powers a rotary air compressor and is reasonable for their restricted range. Once you introduce gunpowder, they start sniping and that is OP. The idea of a cooldown is ok, but it is unnecessary because if you build something that requires a cooldown it is obviously firing too fast. You would instead decrease the rate of fire and a better built one could handle higher rates of fire.
I don't agree that the junk turret is OP. It can be very effective if you as a player decide to mod them up, perk them up, and place them in places where they are the most effective. When clearing POIs, some have places where placing them is a good idea. Other POIs don't have many good places at all. A lot depends on your settings too. If you have higher difficulty with boosted zed speeds, those junk turrets aren't going to help you much in looting and it gets even harder to use them effectively as your gamestage goes up and the zeds become more feral, more green. They give the impression of being OP the first few weeks, but then they die hard in that regard. If you are using them late game to clear POIs and you are finding it OP, it probably means that you've mastered using it. You got good at it, accept your success and pat yourself on your back... you deserve it.
The only way I can see them being OP is when used for base defense when you are already cheesing the AI to loop in front of them so that you can just stand there and reload them. In that regard, ANY weapon you use to damage zombies is OP. The problem there is the AI, not the junk turret.
I have had games where I have found 2 of them in one crate on day 1. I have had games where a week goes by before finding a single level 1. So I don't really agree that they are too abundant. It seems on par with just about any other low level weapon. However, I would be down with the idea of them already existing and set up randomly in some POIs and you have to dismantle them before claiming as your own... because that's a new challenge and interesting game play.