Old topic, but I wanted to correct some assumptions that are untrue.
Promise you this: A King G-Bugle would play as in-tune as the player's ability could support.
Listen to Star of Indiana's '93 show for an example of G bugles being played beautifully on challenging material.
The aspect of G versus Bb instruments that is not often discussed is the most important. The TESSITURA of the music didn't change for the differently pitched hornlines. The OVERTONES of this same tessitura did, however. For the same basic musical line, a G-bugle was usually playing on the next higher overtone. That had much more substantial implications for the ensemble sound than just the pitch of the horn.
The reason that G-bugles gave that exciting, ensemble blend was the differing fundamental CORE of a given pitch, played at that higher overtone. And I think that is why some casual listeners, particularly those who are young enough to have never heard a top-tier hornline on G-bugles, perceive this as sounding out-of-tune.
The best I can describe it is that at the same given pitch (give or take a half step for bugle-friendly key signatures), a Bb trumpet will sound more "sizzly," because its sound actually contains MORE upper partials at any given pitch. The G bugle will have FEWER upper partials at the same pitch, because the pitch itself is being sounded one partial "up" from the trumpet.
You can even simulate it with a Bb trumpet, simply by taping the third valve down. While the bore shape will still be more cylindrical, if you play a g-bugle part on that trumpet, you will definitely hear a difference in the overtone series for any given line.
THAT is the primary difference. And it's huge.