I would say none of them are bad—they aren't unusable or anything—but they each lack what I think of as the distinctive sound of the instrument. The flugelhorns I know less well, never having played in an ensemble that used them regularly, but the bits I heard lack the mellow, conical sound I associate with the instrument. The euphonium was similar and this is an instrument I'm well acquainted with. But especially in the upper range where it is often used for solos it lacked the character, which I would describe as almost velvety, I want. I noticed this in the Holst example upthread as well.Can you put your finger on what you don't like about those instruments? I'm pretty delighted with all three, ecstatic with the flugelhorns actually.
In the examples, the trombones sound more to me like generic low brass than distinctly trombones. Compared to the trumpet, horn, and tuba in particular, they just seem undistinguished. I would be interested in hearing how well the trombones can snarl as well as a swell played by the trombones and the horns back to back, both in the mid-upper range in close position and then in the lower-mid range in open spacing. And then doing something like the big trombone entrance of the Pilgrim's Chorus from the Tannhäuser Overture. Does the distinctive coloring of the trombone emerge? When Audiobro does the closer look with the trombone, I'm hopeful it just happened to be the examples and the distinctive character is in the samples.