I'm actually going to have an opinion on this one - but I'll try to present the facts.
Barb wire or "bob wire" is wire fencing with points on it. The idea is that animals will respect it after touching it once (electric fencing, of course, works on a similar concept).
Barb wire is often the only thing that will stop cattle, who tend to walk right over fences. However, many people say it should never be used with horses. Others state boldly that they've used it for years or decades without a problem.
The problem comes in the psychological difference between horses and cows. A cow that gets stuck in a fence will wait patiently for somebody to get it unstuck. Most horses will panic if they get stuck. That means that they can cut themselves up very badly indeed on barb wire.
The people who have no problem? Horses rarely get themselves stuck in fences (although I do know one mare who managed to injure herself severely twice in six months on the fence - which was board fencing). They respect fencing so, yes, it's possible to have a herd in barb wire for 40 years with no accidents.
However, the accidents tend to be more serious when they do happen. I personally would never use barb wire with horses (unless co-grazing with cattle, in which case the risk of the cows getting out would be more serious). But I'm not going to tell people more than that I don't approve of it. I've had a tangle with a barb wire fence myself and was lucky not to need stitches, so...