If you let the racers know up front to bring their helmets to sign-up it is not a hassle at all. Let them know ahead of time via Pitracer, in addition to a hand-out at the gate and PA announcements. WRMC does it for flat track, harescrambles and MX, and 95% bring their helmets with them to sign-up.
Keep a wet paper towel at sign-up to wipe muddy helmets. As far as accuracy, I would say 99%.
Now don't let me over simplify this. In addition to the start-up cost it would be a group effort to run this system every race. You would typically hang four scanners at the finish, and run cables to a computer. This is the same computer that you use to run sign-up, so there has to be a time period to finish sign-up order entry, come up with a race order, print the race order and gate pick sheets, shut down the system, take it to the finish line and hook up to the scanners. This same computer needs to print results of each moto quickly before the next moto starts, have someone run those sheets up to sign-up again.
I'm sure there is a way to run multiple computers but they all must connect to the same database, which means you have to have wireless available. The first races of the season will cause sign-up to be very slow, as you have to enter all of the riders' info into the database they first time they race. Subsequent races they will only need to scan their helmet at sign-up for their info to come up. If they had to get a new tag somewhere during the season it would need reassigned to their current database file.
Definitely not a one man job, but a small army of workers to pull this off each week.
Having a late sign-up after the race order and gate picks have already been assigned throws a real wrench in the works. And of course you will always have a couple racers who say that the transponder results are not correct. The person at sign-up or trophy counter only have a results sheet to go off. They then have to go back through the manual scoring back-up to verify, which in most cases, the transponders are correct (99% accurate).
Live scoring is possible, like they do at Loretta's, but you better have someone on staff with a computer science degree to figure all of that out. Pretty slick system but there is so much that goes on behind the scenes that most aren't aware of. I like it but I can definitely see the argument to stick with manual scoring. Quick and easy.