In my experience the main problem is enforcement. No tampering with the seat belt is allowed. So non-approved adjusters that would make the seat belt comfortable also make it illegal.
If the users response is to point out they do not care about illegal then those adapters may be the best, easiest, cheapest solution.
If the users response is to point out that the adapters may not work in a crash then I would point out the seat belts have to fit properly and comfortably to work as advertised. Also the seat belt you have is not as effective as you think, particularly in roll overs or severe crashes. Even the NHTSA tries to concentrate on lives saved rather than seat belt performance in crashes.
If a person is concerned about seat belt performance and their current seat belt does not fit properly they are left with few legal options.
Personally I have never advocated being dead right so I would suggest making whatever modifications are required to be comfortable and safe. The best solution I've seen had a 4 point harness, with an optional 5th point. It passed inspection because the original seat and seat belt were in place and provided the original seat belt was worn under the harness would be legal to use. The law really is an azz but whatever it takes. BTW the harness can then be used to hold the 3 point in place so it could be more comfortable.
Speaking of legal stupidity, in at least some areas it is illegal to wear a helmet while driving a car. Considering the #1 cause of death in car crashes is head injuries drivers, like motorcycle riders, should be required to wear helmets but instead wearing a helmet can get you ticketed. Something I found out first hand when I got off a motorcycle and jumped into a car to take it to a friends house. I was promptly pulled over and told to remove the helmet. Since then I've tried it a couple times and while the length of time for enforcement to kick in varies it always has.
A co-worker suffered a very serious head injury in a very minor crash. Working with him and watching him struggle with even basic mental tasks year after year was a constant reminder that it is the individual that pays and pays and pays regardless of what the law says or doesn't say.
IMO do whatever makes you feel comfortable and work around the legal as best you can.