sur le plan + adjective — {intellectuel}

{or}: sur le plan + de + definite article + noun — {de la satisfaction}

This is a commonly used set expression that more or less corresponds to "from an X(intellectual) point of view" or "on an X(intellectual) level" in English.

In some instances, you could indeed get away with simply translating it into "~~~ly" such as "intellectually", but it can also be misleading and insufficient at times.

For instance, "de façon/manière intellectuelle" too translates as "intellectually", but you cannot replace "sur le plan intellectuel" in your example with "de façon/manière intellectuelle" that essentially means "in an X(intellectual) way".

They seem similar on the surface, but actually mean entirely different things.

The adverb "intellectuellement" alone does not strictly serve as the equivalent of "sur le plan intellectuel" in your example, though you can still say "se révéler intellectuellement décevant".

You can also substitute the expression "(X)intellectuellement parlant" for "sur le plan intellectuel".