The question is: can formations that disembark from a vehicle that planet falls sustain fire?

There is no restriction on the kind of action the formation can take in the rule. (The only thing that's mentioned is that if the formation ends up out of formation then it has to take an action that
allows it to move so that it can move back into a legal formation.)

It's further clarified in the FAQ



(My underscoring)

cheers!

The answer is yes, and no

In the original sense, planetfall is a mechanic soley used to get troops on the table. The activation is actually made by the Spacecraft, not the formations doing the Planetfall, and Spacecraft cannot 'sustain fire'. Once you have the formations on the table, at some other point in the same turn they may activate like any other formation, including using Sustained fire.

A very small number of formations can 'Self planetfall'; they are deemed space capable and can arrive without the need of a spacecraft. These declare one of the usual activations while still off-table and if successfull, they planetfall onto the table in the pre-plotted location, scatter as usual, and then carry out the nominated action - so these can declare a 'sustained fire' activation while still in space and activate to arrive and carry it out.