The question whether flat or domed buttons are preferable has popped up again at the concertina.net forum. Some individual testimonies have been presented for and against various designs and naturally individual playing habits may influence the preference but the most important matter is avoided as usual, namely that seemingly pure conservatism obstructs a healthy reformation of the keyboard measures and a number of design features including button diameter and profile.
The British origin concertinas traditionally have had 4-6mm diam buttons while German origin ones have had 7-13mm diam.
For other keyboard devices like typewriters, computers, telephones, calculators etc prefered button size usually is 10-15mm with a flat or concave design which no doubt is more comfortable than the small traditional British style concertina buttons.
The general instrument construction sets some limits but minor construction modifications would make application of more functional keyboard sizes possible as well. I find it a bit sad that so little is done to improve the playing conditions for the concertina which no doubt has a potential for being one of the most "user-friendly" of all musical instruments.