Don has the right of 75 amps for the AFCI. Realize that the 75 amp is only there for a very short period and a regular circuit breaker will never trip. There is literature on the testing and UL has a video about this.
Also you are correct on that the indicator is not a testor, they cannot safely build a tester that will handle the proper signal and the false signal the indicators send may not even trip all AFCI breakers even within inches of the breaker. They are a tool that will many times work but not always. The only true test of one of these is the test button on the device.
Yes there can be length limitations to the AFCI, but it is not a limitation in the device, it is based on the impedence of the circuit. If the length of the circuit to the fault is great enough, a fault will not create a 75 amp or larger signal. In typical house wiring this is not as great a problem as it would seem as we are not relying on only this type of signal. In a long circuit to fault condition eventually in a system with a ground you will arc to ground and also trip the AFCI (similar to equipment ground fault protection).
The Combination AFCI (not Cutler-Hammers AFCI/GFCI combo) which checks for both series and parallel arcing is now UL listed and available from two manufacturers - Square D and a Korean company. Look it up on the UL website.
I thought they were snake oil also, and so I got educated and now believe in them. The manufacturers have been very careful to avoid the problems that the original GFCI devices had.
One possible note of caution is that they are being tested to see if they may have problems operating on modified sine wave inverters. These would be in use in residences run on solar power if the owner did not want to shell out money for a good true sine wave inverter. Realize that this is not proven yet, just possibly suspected and being tested by UL. I am just giving you information in case you do DC to AC conversion work. This to me is a testament to the great testing these things have gone through.
Lastly I am not an engineer who designed these or work for any of these companies. Sorry for the long post. I hope it reads correctly without to many typos as I've got to get back to work.
Shane, P.E.
