Tootler
Brass reeds were a feature of quite old instruments and you're right; they are quieter and, to some, sweeter. I can't comment on whether they're more fragile except to say that Lachenal used his "Steel Reeds" stamp as a marketing suggestion of a superior instrument. To my ear steel reeds are louder and have a brighter tone.

If you're playing a concer in a band context I suspect brass reeds would be lost unless you're solo and well miked. The best Wheatstones, Lachenals and Jeffries all have steel reeds (there's a certain rumour to the effect that postWWI Jeffries couldn't get the same tone as the prewar ones because the grade of steel was no longer available) and the fact that all these good concers all had steel reeds is probably not accidental.