One possible problem scenario with a usb attached drive might be if the computer is both a server and streamer and the USB drive attached results in the computer producing more noise that introduces jitter in the streamer to DAC interface.
That is a unique case where a single device serves as both music server and streamer. I can only speculate that might possibly be a factor in some cases but even if so should be manageable with a good design.
I use USB drives on my music server PC but I have separate streamer devices that connect wirelessly to the server. This is a very good configuration IMHO because the streamer is far away from the home computer with no vulnerable wired connection and home computers can certainly introduce noise capable of producing jitter on the feed to a DAC.
So it is always possible in this case one connection behaves better than another due to noise differences but even if so that does not mean that one approach is better than the other categorically. That would be jumping to conclusions.
Also note that most good quality modern DACS are designed to be jitter resistant so the likelihood of an audible issue is even less in that case.
