There is a Facebook group called “Hancocks and Blue Valentines” where this occasionally comes up and is discussed. I own a Hancock gelding --great horse. However, I would hesitate to buy ANY horse that is overly inbred (or any critter at all). There has been some discussion lately on the Facebook group about ringbone showing up in very young (5-6-7 year old) Hancocks who are heavily inbred. Of course, no research, all antidotal.
My other concern would be “hot and reactionary” – from what I’ve seen of Hancock horses, they are as a group, in my experience, more on the stoic, unflappable side. I can only speak from my own observation of 5-6 Hancock horses I have seen personally --mares and geldings. As a group, while none were “hot and reactionary.” The geldings I’ve met have been like mine, hard working, slow to learn new things, but once they learn them, never forget. My horse is unaffectionate, even contemptable of humans --but he will do his job perfectly --I’ve had him almost 20 years. He has never nickered to me, or sought affection --prefers other horses to me --but carried me 250 miles on a ride across MI and never missed a step --maybe he’s still mad about that. He is still my go-to horse in any new or difficult situation. On the hunt field, he’s the horse that is asked to pony or ride beside any fractious youngster (human or horse). He jumps like a deer --and falls asleep at checks. The Hancock mares I’ve met are hugely loyal to their owners, and occasionally are jealous and over protective (chase other horses away from owner). I’ve been told they have more “bottom” than any gelding ever (don’t know, don’t own mares).
I would own a Hancock in a minute --but not overly inbred or “hot and reactive.” I actually do have a second Hancock sort of --he has a Hancock a few generations back --but gave him the curly hair and roan color --but he’s heavily Freckles Playboy and Poco Leibra (I think that’s where the Hancock comes from).