Since I started racing I haven't experienced a day where there was an issue with either RACE or CSBK using the AMB system.
The in-track loop on the Fabi track didn't pick up my transponder the last time I rode a RACE round there. The year before that, it was intermittent. I wasn't the only one with trouble that day. It doesn't seem to be a problem when using the front track (which is every other track configuration).
My main beef with it is that it's expensive, and now AMB has gone to a subscription model, which means the per-event cost is even more. It is at the point now where for someone only doing two or three events per season (a lot of VRRA members are like that) it's cheaper to rent one per weekend. (CSBK does this by default, it's included in your entry fee.) At least if you rent it and the transponder has a problem, it's someone else's problem - "doesn't work, give me another one".
"but it's only a small piece of the total cost of racing" ... indeed. The straw is only a small piece of the weight that broke the camel's back, too.
SOAR started out using an RFID-tag system ... and then TagHeuer bought out the company and shut them down. They're planning to use another RFID-tag system which was being tested last year.