It's not intended for high fidelity (music or full rich sound). It's a communications speaker that is intentionally designed to sound flat and yes, even a little crappy as you say. The frequency response of a perfect speaker for music might be 20Hz-20kHz! The range of the Uniden BC15 is 500Hz-5kHz. This makes it sound sharp and clear specifically to pass the most intelligable portion of the human voice.
That's the problem with descriptions like "crappy". Audio sounds different to different people. What sounds tinny to person A sounds bassy to person B.
Next mix in some older folk who have lost the upper part of their range and everything sounds bassy unless it's really tinny. Then that sounds normal to them but tinny to someone with normal hearing.
Then mix in a communications system that is tinny to start with. It will sound even more tinny on some speakers, but normal on one that has more bass response. But that speaker will sound bassy on a properly set up system by comparison.
But, the correct response for communications is 50-3000 Hz. (all else being equal such as a properly set up communications system. On many analog systems you can get away with 300-3000 Hz since audio below 300 Hz is often (but not always) subdued. But for digital you need the full range of 50-3000 Hz.
What audio comes down to is that the speaker audio should sound pretty much exactly like what the user sounds like in person.
Bottom line is that if you don't like the audio of any given receiver, add an equalizer to make it how you like it. (of course, if your hearing favors the bass, a cheap tinny speaker may be just what you need to make things sound 'good'.
As for the Uniden scanners, I've found the mobiles to usually be pretty authentic. Yes, the portables are more tinny, but there is only so much bass you get get from a tiny speaker. Since the SDS is SDR-based, perhaps Uniden can add an equalizer (even basic bass/nominal/tinny) via firmware which likely wasn't possible in past models.