> "Last time we did something with X it blew out by 3 months" - Ignoring the fact that X is now a solved issue

This is software though, if X has actually been done before then it doesn't need to be done again. It is already done.

Task X clearly had the potential to blow out by 3 months, and they are now working on task Y that is similar to X. It is a reasonable position to assume that there are other as-yet-unknown issues that might cause it to blow out by 3 months until someone has demonstrated that all the unknown unknowns are also resolved by doing it quickly. That is just basic evidence based planning.

I've always found that finding a similar scope problem and how long it took is the best predictor of how long the new problem is going to take.