A Gypsy/Haflinger just sounds very... strange? They don't sound like they'd mesh very well, if the stallion and mare weren't really good examples of the breed it'd turn out rather bad. If the Gypsy Stallion wasn't of riding type... it'd make it worse.
I personally like the look and functionality of the Gypsy/Arabian Cross. I am looking to produce a horse of cob type which all the gypsy/arabian horses I have seen on google are. I am psychically disabled to a point need a very calm horse gypsy's are clam and well tempered. Arabian are well tempered too + are extremely smart. I have seen what lack of bone can do to a horse. I have a very talented mare who suffers from horrible arthritis because of her splint bone being fractured. I have chosen a very refined, well breed, small riding type gypsy stallion who has shown in dressage. His head isn't huge, which is something people have expressed worry over, and over again he has roman nose not a true roman head. The mare has good bone, a straight profile and while refined she is sturdy built.
I am sorry people can't see the use of a cob type horse they're very popular in England and is what I need in a horse. I have only seen a handful of them in the states, and only online no where near where I live. And while you have brought up a few valid points you can get a mishmash horse breed two well breed horses of the same breed together.
Lots of vanners in the UK, never seen this cross though...