Almost everyone has always warned against the adapters, but I had never seen any posts of anyone telling of damage to their 5th wheel as a result of using the adapters. However there have been a couple posts fairly recently that have posted about damages caused by the adapters.
Personally I wouldn't use one simply because they do look like they would put unnecessary stress on the 5th wheel pin box area. If it did cause some damage it would definitely not be cheap to fix.
My brother-in-law uses one to pull his 33' 5th wheel with speed boat in tow and has had no issues. But I wouldn't do it.