It is not so much that they are hard to handle as it is that the Dutch tend to train in a way where they build the dog up to be crazy to bite with little control being developed along the way, and then they introduce severe compulsion to get the control they need, which is not that much because the sport's goal is to select for police dogs.  I would argue it is not a smart way to train and they would probably argue their style has produced good results because it weeds out all but the most driven and resilient dogs.  The temperament of KNPV dogs has changed some over the years in that you don't see as many highly genetically dominant dogs.  One reason having to do with the winding down of 9/11 and terrorism and another being, those type of dogs, when imported to places like the US, could not be handled by most K9 handlers and often were returned.  We have quite a few KNPV line Mal X's in our club and they are not hard to handle if trained in a smart way.  Because our club members almost exclusively aim to compete in PSA, establishing control from the beginning is paramount.  One way to do that is to establish a mindset in pups that they need to settle and learn think in obedience and bite work.  Typically food is used when obedience is started because you can use a much higher rate of reinforcement than with a toy and a toy is going to stimulate prey drive more and those lines typically have more than enough prey drive. With bite work, you have to try to keep the pups' minds and drive at a level that they are highly motivated to bite, bite no so high that can't think and learn.  That is very different that the typical Dutch approach where the pups learn bite, bite, bite with no control and then severe compulsion is added latter.  As I said, the level of control needed in PSA vs. KNPV is night and day.  Also, most KNPV trainers will train a pup to a PH 1 level and sell it as a police prospect and get another pup and repeat the process.  In the states, the dog typically is the handler's companion and if they compete in PSA, they want to try to compete at the highest levels and they will keep the dog through its life.  Exceptions are if the dog is lacking in drive or nerves, or if the dog is so extreme, even smart training is going to make success in PSA very difficult.  Even German Shepherds bred for KNPV that come to the states as police dogs tend to have control and handler aggression issues because they breed for extremes and don't introduce control early in the training.  They probably have less stringent requirements for police dogs than police departments in the states.

They have different goals. Won’t they stop buying KNPV dogs if word gets out they are being returned?

I think the trend of breeding highly genetically dominant dogs has significantly decreased and the issue now days is more about the foundation training of green KNPV dogs sold to the states as police dogs.  The other issue is that probably the majority of K9 handlers are young , inexperienced and don't have access to good training especially how to manage dogs with a foundation of no control and some aggression.  A department in our club's area sent a K9 handler to our club's main training director/decoy for help getting the dog certified because the dog sent the handler to the hospital.  There is a saying that many K9 handlers go into that line of police work for the t-shirt.

Green dogs are trendy now. I only learned about them on the other forum about a year ago. If the dogs are capable, doing their own training is a good idea.  It’s very difficult to get a K9 pick in our police departments.  Not everyone who wants to be a handler is chosen.  I’ve noticed the new ones are are on the young side.  My car broke down and a K9 officer was first on the scene but he left his dog in the car.  I talked to him about Mals and training and we had a good chat before the tow truck arrived.  The handler looked about 18 but was probably closer to 30.  

https://youtube.com/shorts/5sn_cf8LwVG?feature=share

https://youtube.com/shorts/5sn_cf8LwVQ?feature=share

The first video isn’t available anymore. The second is awesome! He’s quick and sharp. I love how he flips right into places next to you at the end.  I’m so envious. I wish I had done more with my male dog when he was younger. I knew you did jumps but the tube is new. How did you train him to race through it?

I copied the first link wrong.  Teaching the tunnel is pretty easy for most dogs because most naturally enjoy it.  I simply put him in front of the tunnel, went to the other end and show him his tug and said tunnel and as soon as he exited, we was paid with the tug.  Then you progress to send him and remain at the entrance and time the tossing of the tug so it lands just outside the exit as he exits the tunnel. The service or flip Finnish is just the result of teaching it early with tons of repetitions and only and almost always quickly reinforcing fast comings to heel.  In the level 2, I will enter the trial field with him in a down just behind the entry point with me facing him about 15’ feet away and blow the whistle and he will run to the heel position and we will heel to the first cone of the first exercise.  You are not required to enter the field that way in the 2’s, but it puts the whistle recall and flip Finnish in the dog’s head from the beginning.

Wow... that crawl through the tunnel was fast... he cannot do it standing full, so had to do a partial squat to get through it... and it was quite fast too.

And finally, that 180-degree flip and sit down after he exited the tunnel was done quite fast too. I think he was partially sitting as he was turning!

FWIW 

Out of over 30 dogs in our club, he is by far the largest and one of only two GSDs, so he is pretty fast and agile.  He is running and not crawling.  He is extended when running which lowers his top line and his head is lowered.