He does but that’s not a unique trait in working lines. Depending on the foundation work and how long the dog has been doing it, some dogs do struggle to transition from sleeve to suit. Most dogs I’ve seen do so without much issue. Lower level dogs are the ones I see struggle more.

It seems like a good dog would be able to transition more easily, the more time they spend training anyway.

I think foundation, temperament and type of suit are three main factors.  With my dog, he was started on a leather pillow.  The leather has an advantage over jute or synthetic material because jute makes it easier for a dog's teeth to bite into the jute and a leather pillow is more slick and teaches the dog to bite harder if he doesn't want to lose the prey.  Also we never make much prey  movement with the pillow, the decoy is standing still with the pillow as opposed to moving around to stimulate prey, and we never use whips to stimulate prey because all those approaches become crutches and are not going to be seen in a trial except for flee/escape bites.  Then we transitioned him to the suit.  A Belgian sleeve worn in reverse can be used, but too often, unskilled decoys will feed the dog the sleeve to protect their chest which is not protected by padding as with a suit.  It also helps to have a large, sturdy table with a swivel pole in the center to attach the dog to.  This makes targeting the bicep easier and more accurate to train.  The issues some dogs have is seen when using thin competition suits which is all we use.  With a thin suit as opposed to a thick, puffy suit, the dog can feel the muscles and ligaments moving under the suit and realize they are biting the man and not just equipment, which is what you want because it teaches the dog to fight the man and not just bite equipment. Most good dogs are not bothered by a competition suit and those that are uncomfortable in the beginning often get over it with a few sessions.  Even more nervy dogs can do okay, especially if they have more extreme prey that can masks nerve issues.  The extreme prey compels them to stick the grip even though they are stressed.  I don't think my dog has ever bit a sleeve or maybe has a few times, which turned out to be a good thing because PSA changes their rules in the level 2's allowing a fend off item to be a sleeve.  That creates problems for dogs who cross compete in sleeve and suit sports.  I think it is unfair and doesn't add to testing the dog IMO.  You can also now use a bite suit as a fend off item which I also don't agree with, but I think we have only slipped the suit with my dog one or two times.  I think a fend off item should be a rigid object that physically blocks the dog from biting the upper body.  That would include things like metal trash can lids, an actual wide wooden board with handles or something similar.  I have seen large, strong dogs just push through two metal trash can lids and get a bicep bite anyway which is acceptable.

I think as long as the training matches the temperament it shouldn’t be too hard. The dogs can be a little weird about it if all they’ve every bitten is hard surface items like IGP sleeves and tugs. 

I think it’s an issue that generally can be overcome with a little work. I think a lot of the PSA scenarios are over the top for me. A lot of them ask way more control from the dog then I’d be any in a protection dog. Some of the other ones I think lead to a less safe dog from a sport perspective. While it looks good on video, real world criminals aren’t fending dogs off with sleeves. If there is a sleeve in the picture then that’s what I want my dog to target. If real world protection was his primary duty I’d feel differently. It is a giant setup for a sleeve dog because if the helper slips the sleeve and the dog goes back at him there’s a whole world of problems.

I agree with the over the top comment and the control is counter to what you would expect in a protection dog.  PSA is no more designed to develop protection dogs that any other sport.  KNPV is a little more unique in that respect because it is not focused of precision obedience and control.  Many dogs in PSA do not have the genetics for civil aggression and are super social, sporty dogs.  There is a dog that excels in the sport that is a backyard bred pitbull that probably weighs 35 pounds, but is very correct in the scenarios.  I think part of the issue is that the developers of the sport wanted to create something unique and that would impress people since it is an American sport, but it is not a good predictor of police or protection dogs.  My bias is that genetics is what leads to those types of dogs.  

How is PSA training different from PPD or K9 training?

Being a sport, unlike PPD or K9 ( I assume you mean police dog training) the dog's performance is judged based on standards of the particular sport and the performance is given points by the judge.  Enough points have to be earned to pass.  For example, correct heeling should be focused with correct position.  Sits and downs either in motion or as change of positions earn more points for being fast and correct meaning they are straight, not sitting or downing on one haunch, not moving forward when commanded to change position from a sit to a down, outs should be fast, guards should be fast and focused on the decoy, transports should show the dog coming into the transport fast and focused on the decoy, bite mechanics such as targeting and grip are scored, speed of entries into a bite are scored, and those are just some examples.  If a dogs puts teeth on suit when they are not supposed to, they are disqualified.  There are call offs.  A lot of the level 3 scenarios  require ridiculous control and have nothing to do with what you would expect of a PPD or police dog.  A PPD and police dog just need to accomplish a task and have some control, but the manner in which they do it doesn't really matter.  Many police dogs are pulled off the bite and don't have an out, which would disqualify a dog in PSA and other sports.  There really is little comparison to sport vs. PP or police work.

Yeah, I knew about LE dogs not having an out, but not all of them.  How do PSA and KPNV compare? We don’t have clubs for those here that I know of.

I don’t think they compare at all.  KNPV, while still a sport, is designed to select genetics for police dogs.  Most KNPV dogs get their PH1 and the handler sells them as a police prospect.  There is little desire to train a dog to the highest levels and dogs that squire the highest levels are typically less suited as police dogs.  It is a totally different culture.  The dogs are made crazy to bite and then severe compulsion is used to get the PH1.  KNPV got into trouble a few years ago when someone put cameras out that were hidden to show the abusive training.  The other side of that is that the abusive training washes out all but the hardest dogs.  The obedience in KNPV does not require a high level of precision and handlers are not attached to their dogs and typically train a dog to a PH 1 and sell the dog as a police prospect and get another dog.  Some of the training can be brutal.  I know of one KNPV dog imported to the states that threw great genetics but could not work anymore due to a damaged larynx.  The trend has changed from lines that produced super genetically dominant dogs to super social but extreme prey dogs because the older style highly dominant dogs sold to other countries as police prospects did not have skilled enough handlers to deal with such dogs so the market demand changed.  Some of the old KNPV lines, which outcrossed to other breeds like pit bulls, Rottweilers, Great Danes, we’re very serious dogs.  Modern KNPV lines are still considered Mal X’s because they are not purebred and don’t have papers.  If the dog is fawn colored it is called a Mal X and if it is brindle it is called a DS X.  Dutch FCI registered dogs are different and generally are not as strong as the unregistered cross bred dogs.