I'm sure we'll get a bunch of welcomed opinions
My personal experience the past 15 years in tuning and playing close to a hundred brass reed concertinas is that the quality of the reeds varies quite a bit. The mahogany, english, entree level Lachenals could have the stamped mass produced reeds and not be the greatest players. However, I've come across examples that played well with decent volume and response rivaling the lower end steel reed models.
The 20b anglos are all over the place Some are great, where you thought you were playing good steel reeds. Some had reeds with terrible tolerances with gaps between the tongue and shoe that you thought you might be able to drive a truck through. All played, which was probably Lachenal's goal albeit some better than others.
As you move up into the rosewood english models I've found the reeds, as you might expect, generally improve. I've played some sweet, responsive examples. But I've also found some that seemed to have "bricks " for reed tongues.
Early Wheatstone reeds can be nice, but again vary widely.
I've found that once Chidley took over at Wheatstone and began producing brass rivet reed concertinas the overall response and sound of these instruments is very good to excellent.
So there seem to be general trends with some exceptions which leads me to advise folks to evaluate any concertina on its individual merits. (Some wise person in this forum once pointed out that the individual condition and repair history of a concertina over 100 years can be as or more important than its model ranking or maker's pedigree.) Was that you, Malcolm Clapp?
A word about brass reed instruments staying in tune
My experience is that brass reed instruments stay relatively in good tune AS LONG AS played within the limits of the instrument. By that I mean you can't take a brass reed instrument to a session and play as loud as you can, pushing the instrument to an extreme and expect it to stay in tune. In this regard steel reed instruments are much more tolerant of abuse. I've found that brass stays in tune if you respect the volume capabilities of the instrument.
It is true that brass reeds are not as resilient as steel. A brass reed, not carefully filed can develop stress fractures and fail. This is not widespread in my experience but occasionally does happen.
To my ear brass reeds can have a certain unique sound and charm. The overtones seem less harsh and the chords are sweeter. You may give up some volume and sometimes a bit of quickness but there can be some compensation in tone with brass.
One man's experience and opinion