That's definitely not true. The players might have many different features that affects the sound and playback. For example: gapless playback, support for lossless or other non standard codecs, stereo expander, different kinds of EQ, crossfade, different DSP effects like reverb, crossfeed, Ambiophonics R.A.C.E., preamp/gain control, clip control, replay gain.... Those were some of the musical features. Then there are other features that might be important: database or folder playback, or both, does it support album artist tags, or multi disc albums, lyrics support, album art download, FM scrobble, streaming support, metadata lookup, social media communication, etc.
The GUI is the least important IMO. I don't care if it is ugly as hell. The most important is the musical features.