I think the key is the execution of the technique.. a properly built and engineered cored hull is going to be stiffer, stronger and lighter than solid glass. The problem is knowing if the boat was truly properly built.. this means good lamination techniques and properly isolating any through hull locations with solid material to avoid risk of leakage into any coring.
I tend to agree with the 'no core below the waterline' idea too. I feel that any blistering that might occur is a bigger problem with the relatively thin-skinned cored hull.
I agree. Much more stiffer and lighter.
It is very important that you have a light boat over the waterline. This will provide with the same ballast a much better stability with relevance for the final stability, or safety stability, as it is sometimes called.
If you have a proper built cored hull with reinforcements where they should be, it would be as strong to impact as a solid fiberglass hull. Open 60's (and almost all race boats) use cored hulls and they are more stressed in some years of use than any cruising boat in a lifetime and there are several that are still around after 15 years of intensive use (and an incredible millage and many circumnavigations). They had their more than fair share of impacts, some so violent that had pulled their keels of, without breaking the hull.
The real problem is price. An underwater cored hull strong and reinforced is much more expensive than a solid fiberglass hull, so most boat builders just use cored hulls above waterline and on deck (where it does not need to be so strong and so expensive and where its use is more important) and use the less expensive solid fiberglass hull underwater. The solid fiberglassed hull will be, for the same impact resistance much cheaper than a strong cored hull.
That's why only really expensive boats use cored hulls all-around.