Random number generators using a GPIO pin?
The self-evident difference between using a digital input, and an analog one, is that there's a greater span of seed values possible in analog.

Second, but perhaps more crucial: If the digital pin's floating "median" value is not precisely midway between logic sense levels (which are not the power and ground rail levels, but switching thresholds for the input circuitry) the resultant bit stream will have a strong bias towards 1 or 0. This bias skews seeds much more strongly than a skew in analog levels would.

It would take very little coupling, either from nearby traces or signals, or through some resistive pull-up / pull-down, whether designed or accidental, to push a "floating" digital pin to unfloat - and that could as well happen post-deployment. In engineering any design, this form of failure mode is best avoided.