I don't see a ewe necked horse, he's just a little shy on work and muscle.
As always, 'it depends'. If you're not in control of a horse and you put yourself in a bad situation, where you need control of the horse...yeah that can work out very badly.
I've had horses that were quite a ways 'above' me. A little longeing, a few riding lessons, not feeding quite so much rich hay and bagged feeds (without letting 'em get thin or poorly nourished), maybe sticking to riding in a paddock with the gate shut for a while, and then gradually going from a little walk back to the barn, to a little tour around the property, to a trail ride accompanied by other riders who are quiet and go at a steady slow pace, and consider what I can and can't do...or shouldn't do...LOL....
There are ways of being careful, of being cagey and making things work. And of just being patient and disciplined. Most of my friends, and myself, we don't rush things, we give it time, and are willing to practice, get riding lessons, set up the situation so that we win, not lose.
Not everyone can do that, though. Some have no patience for ring work. A good many people don't get the connection between lessons and exercises done as part of ring work, and controlling your horse out on the trail, for example. They don't want to spend time practicing changes of speed and turning in the paddock, pen or riding arena, they don't feel they need lessons.
They'll often wind up getting a very fast ride back to the barn or a nasty fall, when they decide to go out in that nice big field or down that pretty trail, before they've got the tools to do so.
Some of it's about 'program'. A half hour ride daily or 6 days a week, week in, week out, can often get the friskiest horse settled and working better - a ride once a month tends to work out - pretty bad, unless it's a very, very quiet, older horse (or a breed that generally tends to be rather quiet overall...but even then they can have a bad day, and are more likely to when not in a 'program').
On the other hand, to take a horse that has had very little training and you not having ridden for a long time, and deciding to go on a long trail ride with some advanced riders who love to go fast or by yourself and expecting to be able to handle whatever comes up...maybe not so cool.
I can't count how many times I've gone on a trail ride with someone who hadn't been working their horse regular. Invariably there'd be a big problem - the horse wouldn't be in shape and he'd be exhausted (or lame the next day), he'd get mad because he wasn't used to being worked, the rider wouldn't be able to control him, and eventually someone would be walking back to the barn rubbing the muddy seat of their pants....