I once was able to test them
... they are almost comically bad.
I mean one of the main tasks of such devices is to capture 20ms of audio samples, package them into a RTP frame and then send them. There's more to building a good ATA, but that's the most basic thing you need to do.
Of course, since they are running normal operating systems, there is a bit of jitter to be expected, so if you compare the expected time of arrival to the actual time of arrival you should see some noise, in a histogram that should resemble a normal distribution close to zero. So the error is more likely to be between 0.1 and 0.2 ms than between 10.1 and 10.2 ms. It may be overlaid by a steady rise or fall because your sampling rate is not precise, but any half decent ATA will compensate for that.
The adapter I've been able to test had a "box shaped" distribution... and to make matters worse, the culminated error rose in discrete steps. Each one of those steps means that not only you will have a glitch in your audio, but also very likely that your modem connection will break. I have no idea how on earth they managed to make their device that bad.