My understanding with harnesses on OEM cars is that they don't all any movement, possibly causing injury if the roof were to collapse, but most of all, seatbelts have shear brackets, that give in case of an impact, taking in xx% of the impact force. Harnesses are mounted rigid and transfer all of the impact to you. No biggie for a racer with Hans neck brace, not quite as safe for a soccer mom.

There again ... everyone drove without seatbelts 40 years back ...

 

I got 4 pointers in my CGT
I love them, but Im aware if I get in an accident Ill be pulling a "Dale Earnhardt"

body stays in place, head and neck keep going. The end result may not be pretty.....

Im betting the DOTs disaproval lies somewhere in that.

 

The safety issue has less to do with neck movement (if you hit something hard enough in a street car for a HANS to save your butt, something else will kill you anyway) and more to do with spinal compression in a rollover. With a conventional 3-point belt, your torso can move. With 4+ point belt, it cannot, so if you put a car on its lid while wearing them and it doesn't have a roll bar/cage to prevent the roof from collapsing, the effect will be similar to jumping off a small building and landing on the top of your head.

e.g. not pretty.

For the majority of situations, you're not really going to be better or worse off wearing them. There are a couple though in which you'll die in an otherwise survivable accident. I suppose it's up to everyone to decide what's an acceptable risk for them.

So far as DOT approval goes, virtually all 4+ point harnesses are not DOT approved, largely because the process of approval is extremely expensive and time consuming, and just about everyone is in the business of making them for race cars in which FIA, et al, certification matters, and DOT does not as they have no jurisdiction over closed circuits. The various companies that make "tuning" street-centric harnesses just slap a "for off-road use only" label on them and call it a day.