They have some significant disadvantages vs. in line conveyor detectors IMO. I use them on bagging systems and the comments below are based on that.
Firstly they don't have a reject. They stop and alarm. What I'd advise as well though is that you link that to a double bag system. So when it stops and alarms with a positive detection, a double length bag is produced. It at least means it's less likely to go to consumer as it should be taken off by checkweighers (overlength) later on the line.
Secondly test mechanisms are not ideal. The best option is dropping a ball in the flow but it takes careful management not to lose it or have it damage jaws. What many sites do is have a food safe plastic "string" so they trigger the detector but remove the test piece. For my mind it doesn't test the timing but auditors appear to accept it.
Thirdly, because the test stops the line you then either then have to restart it, or you do your second and third checks with no product. Yeah. Bad practices all round!
So that's all the bad...
On the upside, if you can get around the above problems, throat metal detectors are cheap. Your packaging can be metallic with no issue as you're detecting just before wrapping. While x-rays or higher end metal detectors can cope with this, they cost more. The sensitivity is often pretty good and they can cope with high throughputs. Also while all of the above issues are a problem, having worked with them, I've also had fewer complaints come back later. That might seem baffling but while it's hard to test the timing; it's also unlikely to be wrong on most packaging systems you're using these in. You normally couple these below a multi head weigher and when the product falls will be when the test piece falls if you put it in flow as it's about when the cup opens. What might scupper your jaws is if that test piece is more dense than your product of course but it's less prone to tinkering. What you can't do is mess around with a reject arm or air jet. Or mess around with reject timer settings on a belt system. I bet you that 90% of the metal complaints I've had in my career have been at least in part due to someone messing with a reject system on a conveyor metal detector and that's really not very easy to do.
So while I had and still do have my reservations, actually they're not bad.