I used one for 5 years and never had any problems with it.
That being said, for the last few years I have been towing the trailer in my sig with the Companion in a short box with the pin still 2 inches in front of the rear axle and have not hit my cab yet, even backing into 90* spots. Can I hit it, yes I can. Don't want to of course. I would run the Companion before I ran the adapter.
Me personally, there is no way in hell I would run a gooseneck adapter on a 40 foot long trailer that probably grosses around 15k pounds. It is just too much weight to chance it. The adapter turns into a foot and a half long cheater bar of leverage against your trailer frame. You may also run into problems with it chucking real bad.
The fifth wheel I had the adapter on weighed 7500 pounds loaded.
Chances are you could get away with the adapter, but a cracked frame would suck to have to deal with getting fixed, even though I have never seen a documented failure.
What you really need is a slider hitch. There are two or three that are made to work with the B&W hitch you already have. I would recommend the Colibert slider. Google it.
Sorry to be so wishy washy.
1999 Ford Superduty F250 PSD CC SB 6spd 4x4.
B&W goosneck/companion hitch, Airlift 5000 airbags.
2006 Sierra F28 Rear Kitchen 5th Wheel, 31' = 10k pounds.
Nights camped in 09-14, 2010-23