I have to disagree with a couple of things here:
Not really. This is well within the capability of a good tuba and euphonium players. Circus marches from the turn of the 20th Century have more demanding parts, especially for euphonium. See The Melody Shop, The Circus Bee, Entry of the Gladiators (when played at circus tempo), Barnum and Bailey's Favorite, etc.
Crossed voices are nearly unavoidable in writing for low brass ensemble. The compass of the instruments involved is such that the bass clef will inevitably get crowded with more than a two voices. The physical nature of brass playing precludes keeping some voices above the staff for a protracted period to create more open spacing. Therefore, it's not a matter of if the composer crosses voices, but how well they do it. Most of the time, the voice crossings are pretty clean. They're either two different lines with distinct identities or a single active line moving through a static chord.
That said, the voice crossing at m. 17, beat 3 does sound awkward to me with the B-natural and B-flat colliding. In fact, to my ear, the B-natural in that figure is a trouble maker all the way through m. 23, because it's either colliding directly with a B-flat or causing a cross relation with one. Even in the contemporary idiom, it was jumping out at me.