Reviving a bit of an old thread, sorry.
I'd say they're on the rise. There are now at least 4 in Arkansas, including the newly formed Arkansas Musicworks Brass Band who won the US Open in 2018. I'm the executive director for that group but we actually have two in Northwest Arkansas, one in Little Rock, and one in Northeast Arkansas as well.
Fountain City (Kansas City) has had the most competitive success internationally, recently or ever(?). They've also got multiple feeder bands as well.
BTW, our band is looking for a fill-in euph player to come to two competitions with us this year. Please message me if you know of someone that may be interested.