Maybe they're just too common. Playing on default difficulty, 50% loot, and not going out of my way to find them, I've stumbled on four, level one junk turrets after the first couple weeks. I was kind of like, I guess I'm doing a turret build now. It was more than another week after that until I could even get a bicycle, actively trying every avenue to acquire one, and I'm still short of being able to power anything. So in comparison, these junk turrets felt like stumbling on alien technology, way beyond what I could do myself. In that sense, they haven't 'fit' well.
In terms of balance, they are pretty powerful, but only having one active at a time curbs that power. I find the biggest limitation is the clip size, and having to pick them up to reload them. I could've designed my base to better accommodate reloading and their limited range, though.