I wouldn't say the ABG doesn't have a good sound. Just different.If an acoustic bass does nto have good sound, how is a smaller ubass able to have one?
The Acoustic Bass Guitar (ABG) by virtue of the bronze strings and fully hollow body tends to have a hollowness and yet a ringing snap to the sound. It does fit well into an "acoustic" sound (espeically when amplified) as it cuts pretty well, but it is not loud enough to be heard well in a truly acoustic unamplified setting.
Note that you can change the tone quite a bit by swapping strings. I heard a lovely tone from an ABG with tapewound strings, but it was notably quieter acoustically than with snappy bright bronze strings.
The Ashbory, U-bass, Microbass and other plastic stringed ultra uke/short scales (18-25" scale) have a very round warm tone to them. Can be pretty funky (in a good way) if eq'd right. This is a function of the scale length, and string material. They're very thick strings.
Neither is necessarily better, just different. Each has a unique tone that is not quite like anything else. However, neither is really suited well for unplugged acoustic ensemble playing.
In my personal opinion, completely unplugged, my favorite tone is probably the Gold Tone Microbass. It's very pleasing. However, if I were going to pick a campfire instrument or an instrument for porch playing I'd probably go with a full-scale fretless on the largest body ABG I could find. Then at least I could play along with any soft-handed acoustic guitarrist (or a classical guitarrist) that came along.
As it stands, I gave up on the idea of a porch instrument or dedicated travel bass a while ago. I sold my 24" Solid body uke bass as it just wasn't played much. If I have an acoustic gig (unplugged or true acoustic) I bring my double bass. If I just need a near-acoustic or near-upright sound I have a fretless Wendler that nails that tone plugged in. That does leave me without a bass for camping but I can bring any electric and my Zoom B1on for practice on car trips at least.