Everything, from the fact that Wayland has an integer scaling factor, to the fact that non-integer scaling factors will result in fractional pixel values that will either make no sense—a window cannot be 200.6×300.2 pixels in size—or will result in rendering that is not aligned to the pixel grid, like blurry text or blurry images.
The only way to allow non-integer scaling factor at the toolkit level is for the toolkit to decide the fractions, so it can choose factors that will result in (mostly) aligned results. The other option, which is what every other platform does, is to render at an integer scaling factor and then have the compositor do the scaling, using its own factors.