Are there any additonal complications or design considerations to be made when placing voltage regulators in parallel?
Is it safe to assume that only regulators with explicit mention of this capability in the datasheet should be used in this way?

Yes it is !

In practice, you would need such regulators only in very rare conditions (and I cannot even think of one).

There are many regulators available with different output currents. In practice there is almost always a regulator that will fit your needs.

There are tricks to use standard regulators in parallel but these remain tricks. One of the challenges will be: how do you make sure each regulator takes half the load (share the load equally).