People are still making claims about digital that have not been true for decades. This is just one of them. There used to be problems with jitter on TOSLINK, but then just don't use it. The rare time you may get jitter on SPDIF if poorly implemented, but then just change the cable length. Noise on SPDIF is a potential issue, so buy something with transformer isolated SPDIF. USB can present electrical system noise, so isolate it. Hard to comment on Ethernet noise. Many claims, nothing to indicate any level of testing. Theoretically possible, so if you feel the need add a filter. Computers can have noisy power supplies, but then again, linear supplies create awful harmonics on the AC line with lots of current/power. But you can remote your hardware or buy a filter.
Agreed, items like Innuos are simple, and make sense for many audiophiles. Time is money, and most don't have the experience. A lot of the reticence of using PCs as streaming sources is not just USB, but inability to set up the system so that you got bit perfect, and Windows or OSX was not inserting sample rate conversion, limiting, etc. in the middle.