How are number incrementing arrows for accessibility? I would think they would be bad for keyboard users, but is there any study to back that up?
It depends on how you approach this component. Every component's identity, operation, and state should be available to assistive technology to make it accessible.
When an arrow is on keyboard focus, screen reader should inform the user about the identity of the element (Up arrow) its state (clicked/ unclicked) and operation (press enter to increase the value)
Bit of design suggestions: both up and down arrows should have enough padding around them (minimum of 48px touchpoint) and an 8px gap between these two arrows. so people with motor issues won't accidentally hit the wrong arrow. Also according to the new guideline update of WCAG 2.1, there should be an option to 'pointer cancellation' For example, If the user accidentally clicks the up arrow it shouldn't execute the increment action on mouse / touch down on, rather on mouse up / touch up. so a user can move away from the target If he/she decides to abort the action before executing. (https://www.w3.org/WAI/WCAG21/Understanding/pointer-cancellation.html)
And use the ARIA role alert for increment change, so it will immediately inform the user of a state change.