I do want to add to this thread as it seems very positive overall (and nothing is allowed to be “only” positive, lol), that I would, personally, hesitate to buy a horse with a broken coffin bone unless I was getting a price reduction because of it.
OP - are you who I spoke to the other day? Either way, I just had this conversation. My guy has pretty wicked side bone in both of his front feet (but worse in the foot that had the broken coffin bone). My vet suspects that the origin or greater influence on that foot is a result of the broken coffin bone. I would worry about the [eventual] side effects of injury in regard to side bone and potential arthritis. My guy is a TB who literally jumped around an FEI Grand Prix on his freshly broken coffin bone. Most of my horses would have zero interest in doing so, and likely would have reacted much more strongly to the pain from minute one. I can’t really speak to the long term effects because, again, the horse will jump no matter what’s going on with him.
IMO, a broken coffin bone is not a life altering injury. It’s something that I would expect a horse to make a full recovery from. But it’s also still a pretty major injury and something that could have effects for a long time to come. If you want to ride/show a horse for a few years and then sell it, that might be a moot point. But if you’re planning to keep the horse long term, it would definitely be (at least) a negotiation point for me.