Nothing...
Most are programmable to set the output voltage. Most modules do his with a pot so you can easily change it. The only difference with a fixed output version is that it uses a soldered on resistor. But to the dc-dc chip it is no difference. If you want to use a module with a fixed voltage you can remove to pot and replace it with the right resistor. Or just don't bother, set the right voltage and maybe fix the pot in place (nail polish does a great job).
And for the input, non I've seen has a fixed input. They all have a range. For a step down it's some minimum voltage above the output voltage and a max voltage. And for a step up it's some minimum voltage and some max voltage a little under the output voltage.