I own 4 gypsy vanners and have worked with i’d say about 30 others.
Each is an individual, but for the most part…they are quiet and sensible horses. Hard to spook/fluster and truly not the lazy beasts that many think. Yes there are a few on the lazy/slower side, but many are well moving and forward rides. I have also met one that was rather on edge and could buck! They truly want to please, love being with their humans, and are generally fairly confident. Mine would all much rather confront a scary object then run away.
They are COMFORTABLE to ride! Talk about a smooth and easy trot! Not joking in the least, I absolutely adore my stallions trot and many of the others I have ridden. No, you are probably not going to see a bunch of GV’s competing in eventing, but that is not to say that there wont be a few individuals that truly excel at it. And for dressage, I have actually seen quite a few vanners that did/do very well in that ring, as well as hunters and western pleasure. Just because they were originally bred to pull a cart does not mean that they are all a drafty ride and cannot do well in the show ring.
A lot of what makes a good riding vanner and or one that may excel further in riding in a particular discipline, is the breeding. Just like any other breed, there are vanners out there with poor bloodlines, conformation, personality, etc. A Vanner is not a perfect horse just because it is blk/wht and has feather. When buying, one truly needs to do their research and invest in the correct lineage/build…not just any feathered vanner because its pretty. I have noticed that different bloodlines throw horses that are more of a western pleasure/laid back build or dressage/forward and athletic movement. I’d say 2 of my horses are more western pleasure and 2 are dressage/English all the way…love watching that free movement in the shoulder and front end 
I have worked with vanners in FL in 90+ degree heat and they really managed quite well (they handled it better then my NSH did). They were not sweaty and run down, they adjusted just like we do. Have I seen a few horses with scratches and lymphedema, yes. Is it painful for them and hard to manage, yes. Do they all get it, no. Is seemed that a few had lymphedema (only 2 that I worked with) and it was quite painful, but most did not have a problem at all, or if they did, it was very mild. Mites however are something to watch for, though again, they did not all get them.
We are coming out of winter/muddy season (living in MI with my vanners now) here and I cannot wait to give baths to all of my guys to get rid of the muddy feather. so far the only one I have feather issues with is my stallion, whom came to me this way a few months ago. Nothing severe, and I’m sure I can get him all cleaned up as soon as bathing season comes around. So far, all of my GV’s do really well in the Michigan weather an are incredibly hardy.
As for the mane and tail, if you want to keep it long, it is a ton of work/maintenance! but worth it 