The phrase "a difficult theory" is correct and usable in written English.
You can use it when discussing a theory that is complex or challenging to understand.
Example: "The professor presented a difficult theory that left many students confused during the lecture."
Alternatives: "a complex theory" or "a challenging theory".
The strong coupling between the quarks and gluons makes QCD a difficult theory to study.
The Barnsley manager, Simon Davey, said that the quality of the game demonstrated that the gulf between the Premier League and the Championship was narrowing but, putting aside the undoubted commitment of both teams, that was a difficult theory with which to agree.
It should be clear that Descartes' theory of ideas has been, and continues to be, a difficult theory to pin down.
However, this is a difficult theory to disprove.
This is a rather difficult theory.
The effect of a substituent in an organic molecule on rate or equilibrium of reaction involves a fourfold difference between relatively large quantities, a situation which always makes for difficult theory.
The Delaware courts have described breach of the duty of oversight as 'possibly the most difficult theory in corporation law oversight liability upon which a plaintiff may hope to win judgment'.
His meditations on the difficult theory of the point were related to problems encountered in optics, space, and movement; they were published in 1671 under the general title Hypothesis Physica Nova ("New Physical Hypothesis").
Pure Land religion was very approachable in that it eschewed difficult theories and ascetic practices, teaching that in order to achieve rebirth it was necessary only to invoke the name of Amida and dwell on the marks of his divinity.
Though the approach of Cicalese and Leonardi to the quantitative isoperimetric inequality is based on the results of a difficult and deep theory, it has the advantage of providing a short proof that has been successfully applied to several other inequalities, see [1, 18, 19, 24, 47, 48, 74].
While this more ecumenical approach to coercion may succeed in encompassing ways of coercing that the traditional theories would leave out, it creates a difficult challenge for the theory to distinguish coercive from non-coercive proposals.
