A concerto for any instrument requires the orchestra to get out of the soloist's way during the solos.
To a rough approximation, the accordion can be louder than any one string or woodwind instrument, but can get submerged by the string section or several woodwinds in unison, and (just like any string or woodwind concerto) has no chance against the brass.
All 3 movements of the Chaikin linked above are worth a listen (while watching the score, if you can - I think there is a youtube video of the Chaikin with the score displayed.)
If you have a taste for more dissonant modern music, Zolotarev wrote two - he called them 'Sinfonia concertante' rather than 'Bayan concerto'.
Several artists enjoy transcribing concertos for other instruments to play them on accordion. Generally these are examples where the accordion is louder than the original instrument, and if there's a balance problem, it's in the direction of not hearing the accompaniment much.
There are also some number of concertos for bayans and orchestras of Russian folk instruments (other bayans, balalaikas, and so on) rather than western orchestras, but that's quite a different animal - one bayan can stand against a whole army of balalaikas and not be drowned out.