properly mounted 4 or 5 point harness are very safe, they are used in racing etc. they hold you in the seat regardless of point of impact, but they take time to adjust between drivers versus the standard lap belt if your car has multiple drivers. They may not be street legal in your area, some laws are written to help you get out of the car "with assistance" as well as your own safety. (like having outside door handles is a requirement so firemen can open your door in case of an accident.)
matched to a truly bolsters racing seat you will definitely not going anywhere in the instance of a crash, which is good and bad. You will stay in your seat and not be moved which is good, unless the car crushes around you. without the rest of the safety equipment used in association with the racing seat etc, the side airbag and lap belt is not a bad idea because they are both easy to get out of your way in an accident. Especially when the bag deflates.
not legal in texas? yea makes sense havent seen almost no one but people in car shows with harnesses. my husband works with guys that like literally have supped up hondas acuras 240 corollas bimmers mazdas and none have harnesses with their racing seats.
Also you have to look at the build of the harnesses. you want ones with the safety stitch on them, the small folded part thats sewn together for no apparant reason. Those are there to give a small amount of give in an accident, the stitching tears out and allows you move forward a very small amount to soften the impact, but still hold you in place. If you were held right tight in place in an impact, your body would stay there, but your internal organs would still be moving forward, causing organ damage or internal bleeding. And thats just no fun