The platform that an arcade controller works on may be a lead factor in your decision regarding whether or not to buy that controller.
Order your Kombox Impact preassembled with Sanwa buttons or in DIY format, and there is a joystick edition too.
The Atachon BEATdown is an all-key controller that looks like a cassette tape, and it has extra keys for the left pinky & right thumb.
The Haute42 M16 can also be a WASD controller. As for the M13 edition, it has three less buttons than the M16.
Choose one of more than a dozen all-button layouts when ordering the slim and mini SL-X controller from BattleMonkey.
The Punk Workshop Leverless U provides a plethora of features, such as slimness, many layouts, different colors, and custom art.
The nanoGRAM is the size of a card, making it one of the smallest Smash controllers (if not the smallest).
The GRAM PRI$M allows you to play Smash and other titles ergonomically with a variety of possible layouts.
The SGF Faust is a mini stickless controller with LED plus a TRS jack for external input sources like foot pedals.