Think again, my friend. Yes, ad hominem can be fallacious if it is false or irrelevant. But ad hominem is valid so long as it is both true and relevant.

Interestingly, the most common ad hominem fallacy comes from those who have no other rebuttals when a valid ad hominem argument is effectively presented.

If an ad hominem argument fits into all other categories of logic, it is not only valid but devastating.

Practical Ethics

Why there is no argumentum ad hominem fallacy