UK tradition brass band euphonium players expect treble-clef parts that treat the instrument as a Bb transposing instrument. So a written treble clef middle C is performed as an open note (no valves) and sounds a concert Bb a ninth below middle C.
The advantage as far as writing higher-sounding parts is that instead of writing a bass-clef Bb, which is second line up on the bass clef stave, you'd write treble clef middle C, which is first ledger line below the stave.
Treble clef parts like this mean you can exploit the euphonium's considerable range without needing to use other clefs or many ledger lines.
In my experience (UK-based) I've yet to meet a euphonium player who didn't read treble clef transposing parts. Bass-clef reading euph players do exist, but they're rarer. If you find a trombonist who doubles on euph you can give them concert pitch or transposed parts using any clef you like and they'll be able to do the mental gymnastics to play the correct pitch.