USB3 can pump up to 900mA @ 5V = 4.5W, which is pretty good. But each device has its own specs - e.g. LimeSDR has only 10dBm output power, because it doesn't have a built-in amplifier. So if you (1) don't attach a large antenna to it, (2) you operate in your basement, (3) you choose an unused ARFCn and (4) you operate in closed mode, you should be fine form technical perspective. That said, legality of this is up to you - check your local spectrum rules.