BUTT joints are an expedient, fast production method of joining the frames, takes little if any skill and looks "industrial".
Mitered joints take care and skill and do not look like fast production, and industrial quality.
Butt joints are used by the "big 3" Cajun makers, others choose learn the skill and make the effort to go that extra step.
My thoughts are that if you are going to "cut corners" and make (you don't really "make" butt joints) butt joints, what else is the maker compromising.
The black image is a router that produces a miter with a tongue-and-groove centered across the joint.
FYI my 15-yr-old Falcon (Big 3?) has mitered joints. Aesthetics are not a priority with him, though. He focuses on sound and tuning. I dunno if he still miters.
I played only one Falcon.My opinion...
I prefer professionally made accordeons.
There are methods of joining corners superior even to miters in strength and appearance.
There is no wood connection/joint weaker or more simplistic or aesthetically deprived than a butt joint.
Long grain to end grain is the first thing you learn NOT to do in woodworking.
PS I made vernacular furniture for 20 years,, accounts around the US and pieces in Museums in Japan, London, Atlanta, the Hamptons in New York..and in collections across the US and around the world....Why? . Not because I am some kind of genius.. NO butt joints and if that style of work is made with a butt joint, you know a hack made it.
NO I did not, and do not, use butt joints for anything.
Imaging a violin with butt joints.
I find butt joints a sign of disrespect.. just like music books without a spiral binding.. no respect for the buyer/musician.