Although you obviously do not want to elaborate on the specifics, I think it very much depends on what is being discussed.
If he states something as fact, presumably based on a source, and I’m not sure what you mean that you ‘disagree’.
On the other hand, if its a logical point, and you disagree with his reasoning, then it’s no different than any other sefer. One can disagree with something written by an earlier authority (to a point), but must be of a certain caliber for this to have any standing.
And of course, everyone here made the valid point that it would be very wise to think long and hard before concluding that, in your opinion, he is mistaken and you are correct.