I guess I’ll give the other side to it, as have some others here. I’ve owned a few purebreds and quite a few crosses. First of all, whoever said they are not intelligent - that is so NOT true. They are a “slow twitch” type, so they may not respond as quickly, but the ones I’ve had have all been VERY smart. One reason they are so popular in movies is because they are so trainable.
I’ve also found them to be the KINDEST horses ever - the ones I have dealt with were SO sweet, loved people, very tolerant, you could put little kids on them, rank beginners, yet put a good rider on, and you got a whole 'nother gear and performance ability. I fell in love with the breed because I’d never seen such people oriented, soulful horses. I had ridden a lot of Warmbloods, a few Arabians, and several Morgans. Friesians were just - different - so tuned in to their people.
They are generally heat sensitive - although I personally have not known one that had anhydrosis, , they do tend to be heat intolerant. As do some Warmbloods. They are a northern European breed, bred for cold weather. I kept mine clipped year around, which helped a lot, but it is maintenance.
Personally, I found the FPZV/FHS horses much more dressage suited then some of the FHANA horses. One thing to be aware of, the Friesians have kind of followed the Dutch Warmblood breeding model - some are bred for riding, some are bred for driving. It isn’t quite as delineated as the Warmblood model (yet), but when you see sports predicate in the bloodlines, you are probably looking at more of a dressage type breeding.
They are more susceptible to certain health issues - although so are many Warmbloods and Iberian horses. Allergies, reproductive issues, and colic are all more common in the Friesian. But they tend to be SOUND - we use to joke about it at one of the training barns I use to ride at - my Friesian cross and the purebred Friesians were ALWAYS sound. The Warmbloods - seemed at least one or two were always lame, and several were neuro. Interestingly, there were more colic issues with the Warmbloods over the years (I rode there for about 15 years or so, so I saw a lot of horses come and go).
Personally, I LIKE the crosses because less inbreeding = less health issues. I bred several crosses, and they have been great dressage and eventing horses. I had a cross stallion that showed through Grand Prix, and in his late teens, is a young rider’s schoolmaster. One of the crosses I bred is currently in the top rankings at 2nd level. My own personal riding horse is a cross too. I talked to a judge who said she has judged a lot of nice crosses, through the FEI levels. Sometimes you just don’t KNOW it is a Friesian cross.
I know a lot of other breeds that have retired early due to neuro or lameness issues - every breed has its issues. Not discounting the Friesian issues, just pointing out, all breeds and types seem to have their problems, and you have to go into it with eyes open.
Personally, I think they are great dressage horses, but they need to be ridden properly if you want to get up the levels. As a “lower level horse”, they are easy - always round - they are great confidence boosters for beginner riders. But if you want to get them up the levels, you have to get them through their backs - which means riding round and deeper then you might with a Warmblood. And keep them forward and quick behind. As a well known judge and clinician said - training horses is easy, make them do what they don’t do naturally…
Interestingly, while I agree they don’t all have good canters, the ones I’ve known have all had great canters. Anytime you look for a dressage horse (of any breed), of course you need to look at the canter!