I have a small update on the SMD SRX2 (And I'd presume others using this case as I highly doubt they are different inside)
I've had cases of the car loosing connection to the transmitter (Exzes X) and running away at two tracks. Both of these have metal link fencing in close proximity to the track (i.e 2 ish car withs from the edge)
Unfortunaly I do not own a genuine Sanwa internal antena reciver to compare to but I think it might be a limit of the internal antena recivers in general rather than these specific ones and the cars have worked fine with a pair of Sanwa recivers with wire antenas