USB scopes have a place at the bottom end of the markey, but most are a big compromise between usability and bandwidth, as they are both constrained by the USB bus bandwidth and power available. This leads to a scope with limited bandwidth, likely less than 10MHz and a limited input voltage range as well. As well they are connected to the host computer ground, and have noise problems.
With USB3 there is a possibility to have a decent processing engine in the host computer and good bandwidth with having enough power down the cable to provide decent processing and an isolated input section. This will then make a software scope much more useful.
As to using a sound card there are programs out there that do this, often used for SDR as a final decoder, and they are useful if you want to analyse audio, but are limited by the onboard ADC of the sound card and it's noise and sampling rates. Very usable for audio work though with a decent 24 bit 96kHz sound card with low noise. Regular PC sound card will give at best 10 bits of usable data and often only a single sampling rate of 44.1kHz and an often very poor input anti aliasing filter.
