I have a Theory why certain Japanese cabinets used Curved buttons.
Their cabinets likely needed more control panel space for each players arms
Also.. as far as Ive seen... their buttons are either Flat Topped, or Convex (raised bump)
(I personally hate flat or raised buttons. Convex is so much nicer feeling)
Being flat topped would make it less of a problem to press a buttons anywhere on its surface, rather than its center
Add to that, micro-switches that take less activation force.. might allow them to lay their hands flat, while playing
(I cant confirm this, because Ive only owned a Japanese stick.. not their buttons)
But even if all that is true... its still vastly inferior to simply bending your fingers vertically over the buttons...
because you get a superior mechanical advantage. Better strength, speed, and better control.
Again, you never lay your fingers flat, when typing on your keyboard
Another thing that partially may have effected Japanese Cabinets... is that they are Sit Downs. The problem with
a sit down fighting cabinet... is that if you are too short, your hands/wrists might be strained, trying to angle them
upwards on the Control Panel. This is why most US Standing Arcade cabinets have a slight Angle on the Control
panel... to allow people of shorter heights (children), to be able to reach the controls with comfort.. rather than
their forearms being scraped on the sharp front edge of the cabinet.
But As said... even if you were to use all Japanese buttons / parts... Id still recommend using a Straight Line
alignment. Its simply easier to tell where the buttons are, without accidentally hitting the wrong button...
and you get better strength, speed, and control, with slightly bent fingers.
Ohh, and As Ive said before... if you are not planning to use Japanese parts.. you can use wood for the control
panel.. rather than having to have a new metal part cut + drilled. This also means, if you wish to change / remake
a control panel... its that much easier. No need to ever cut metal again.