I've worked on reasonably large >$100M defense programs for the last 25 years. Nearly all of them overran both time and budget constraints. They have mostly been firm-fixed price, except for the last 5ish years, that being incentivized agile. I have seen estimation be accurate when the following criteria are met :

1) Experienced personnel are estimating the work (experienced technically and in the product being developed); 2) The work being estimated is of similar technology to existing product; 3) The time period being estimated is less that 6 months

If any of these are violated, e.g. new hires are given the estimation task, a years worth of work is being estimated, new technology is being estimated etc, i would not trust the estimate, and apply big 'ol bucket of risk money.