There are differences, it's not all smoke an mirrors.
The first time I really noticed a low-quality external converter was when I purchased a replacement from brand X. Their new converter did not sound right to me at all. A few minutes with the scope showed me why, slew rate was so bad it couldn't pass a 1khz square wave without significant slewing.
A client of mine had another box from this company, using it as a microphone input. The analog circuitry was very very bad. Once you got the mic up to the appropriate gain level (for close-up speaking), the noise floor came up to maybe -30dBfs or so. This is one of the cheaper units, around $200.
I have recorded in studios with "the best of the best" in recording gear. There is a lot of pseudo-engineering and trickery around many devices, but there definitely are a lot of shitty intro-level ADCs and DACs. Mandy of these intro-level devices are basically what is inside your PC, including cheap chinese-made switching power supplies near the analog i/o, maximum-compactness PCB layout, etc.
I won't argue about "gold monster cable" or similar smoke, but with these external "sound card boxes" there definitely are differences. Some are noticeable, some only in specific situations would bother anyone, and some are just marketing.