Your premise reflects a major intellectual flaw. Surely it’s not your fault—you’re merely trusting convention.
The flaw is that nobody has really done the intellectual legwork to analyze which particular duties of the presidency somebody like Ted Cruz would be unable to perform.
The answer: there are none.
If this is wrong, you will be able to name those duties, then explain the skills needed to perform them and finally demonstrate how Cruz lacks the particular skills.
The key here—you have to get specific. Otherwise you’re just spouting nonsense.