My Mom owned a horse years ago that was technically a blend of TB, QH and Warmblood. The dam was Holsteiner, sire was a registered QH racehorse, but on paper he was 3/4 Jockey Club TB that had simply been used for QH race breeding. Reviewer was one of the TB names up close in The pedigree - I think he was Ruffians sire as well? Anyway, the TB was pretty alright.
The “QH” stallion was owned by a local cowboy type, who had found the Holsteiner mare at an estate sale situation (along with a few others), and then bred them all to his stud, and sold babies to the local hunter jumper market.
So my Mom bought one of these offspring as a yearling, and I rode him for a few years from 2 to 5, before we sold him. One of my favorite horses ever. Nice well set neck, built uphill, a little too short in the back, but overall fairly rideable and cute. 16.1 or 16.2. Wonderful brain… I actually broke him - super easy guy. He went off for some dressage training, and was schooling 2nd level nicely when we sold him. But his strong point was jumping. He was really easygoing and game for anything. Would have made an awesome event horse, but went to a teenager who was interested in both hunters and jumpers, competed in both, and won a bunch! A really fun project.
There was a filly born as well locally who was 3 years younger than our gelding… Exact same breeding. A friend of my Moms who was an amateur dressage rider, and pretty timid, bought it as a newborn because of how nicely our gelding turned out, especially in terms of his brain and rideability/versatility. She had previous bad luck buying a super fancy papered Warmblood yearling dressage prospect, which ended up a little hotter in temperament than she anticipated, and scared her. But that poor filly was a total throwback to some unknown QH a few generations prior… And with other mismatched parts from the Holsteiner Mom. Short low set neck, built downhill, and 15.2 the last time I saw it, with a really odd looking big barrel and super short back… I never rode it, can’t comment on the temperment. I think my Mom’s friend ended up just giving it away to a really good trail riding home. And buying ANOTHER fancy Warmblood yearling, which promptly scared her… She was pretty determined to bring a young horse along herself, but it was painful to watch. At least she had an easier time selling the failed projects with good papers!
So there you go - a complete case study of unpredictability in that sort of thing, in multiple respects. Our gelding was FABULOUS though -