I don't like it when he takes something that was already published and open to interpretation, and issues a definite opinion about it, because then I can no longer discuss it with other fans without them linking back to the WoB in question and treating it as canon, even though it definitionally is not. There's probably even a WoB that says that WoBs aren't canon, but they still carry more weight among fans than someone else's opinion.
The general term for this in other fandoms is Word of God, and this page gets into why it might not always be a good idea.
That's different altogether than other types of WoB like cheeseplate or "trans people are more likely to become knights radiant," or "Yo I really like Pern you guys. No I mean seriously."
And I try not to read unpublished stuff or anything that might be a spoiler of something that isn't out yet, but I don't think many WoBs genuinely spoiled a future work.