A device communicating over USB3 has to sample once per symbol period, with a 1-bit frequency. Most of the hard stuff is offloaded to purpose-designed silicon, so that 5Gsps analog signal is quickly turned into, say, a 128-bit bus running at 39MHz. That's fairly easy to deal with - especially because most of it is either directly processed and forwarded (like webcams), or has some form of flow control (hard drives).

If you're diagnosing signal quality you're going to want to look at the analog signals, which means sampling at a rate significantly faster than the baud rate, and at a 8-bit or higher resolution to actually see analog behaviour. Suddenly you're dealing with 400Gbps of incoming sampling data - and you have to do realtime analysis on that to trigger at the right time, and be capable of storing at least a few tens of thousands of samples for display.