Some general info about dongles:
The dongles used with pp software or to directly feed RB24 / FR24 are based on SDR (SoftwareDefinedRadio) ICs originally developed to receive digital terrestrial television. Some of these ICs are widebandwidth types able to receive signals from 25 MHz up to 1700 MHz (guaranteed range 422 Hz to 1002MHz). If used with original drivers frequencies not used for DVD broadcast are blocked out.
But there are special drivers available on the net which allows you to access the full bandwidth of the SDR chip. The few wide bandwidth combined with special drivers an be used to receive ADS-B signals. As these chips are massproduction types they are extremely cheap.
Due to the high bandwidth they are less sensitive than a special designed state of the art ADS-B receiver (not the current radar box hardware) possibly capable of GPS generated reception time stamps .
This extremely expensive MLAT supporting Receivers are best for enthusiasts, who do not care much about money. The dongles are good choice for all on/off hobbyists or for secondary or third device receivers.
As Alan said: Eventually, they will have to show color.
Ingo