1 - You can fix the time or the scope of the project, but not both. I can say, "I will deliver project X in 4 months" but you have to be willing to let me toss out scope, and the earlier the better. This does work. Just like I can say, "I will promise to deliver all the scope in project X, I just can't promise how long it will take." Agile helps break this into smaller chunks, and allows you to get more visibility into what's realistic more quickly. I've seen too many waterfall projects slip by 3-6 months at a time only when the final deadline approaches.
2 - I've seen more projects slip because of "Unplanned work" and "Missed Dependencies" than due to "Misestimated work". It's either scope creep, or "I didn't realize we needed to test integration" or "Person X didn't give me what I needed." The only counter-examples I've seen are when engineers aren't consulted in the estimates.