Those high level estimates are what often are so wrong. I think that one of the biggest reasons is that as project team members we’re giving estimates like “2 weeks”. “2 weeks” isn’t really an estimate, it’s a guess, a prediction.
An estimate is something like “1-4 weeks” or “3-5 days”. It really should be a range. One big downside of single-point guesses is that the guess is likely to be treated like a promise. On the other hand, giving a ranged estimate opens the discussion of what could happen (risks) that would push the effort towards the worst case end of the estimate or bring it in towards the best case end.
One of the primary problems is that project managers need a single number to plug into MS Project (or any number of similar tools). That coupled with the observation that these single-point guesses (especially at the high-level) are treated as promises pretty much make a person want to give up on high-level estimation altogether.
If you always give all of your estimates in ranges (e.g. 4-6 person weeks, 1-3 days, 6-9 staff months, etc) it is quite clear that it is an estimate, not a prediction or a promise. Giving a range opens the discussion of what could happen (risks) that would push the effort towards the worst case end of the estimate or bring it in towards the best case end.
Another nice thing about ranged estimates is that when you are asked to estimate something with poorly defined requirements you can give wide ranges to communicate the uncertainty. Well defined, believable requirements get estimates like 5-7 weeks whereas poorly defined, nebulous requirements get estimates like 4-20 weeks. This is especially true when giving high-level estimates.
There’s pretty much no way to get around doing high-level estimates. Your business needs some idea of the investment required to complete a project in order to make trade-offs. Hopefully this happens before your development team spends a bunch of time doing detailed estimates. Also folks in other departments (e.g. marketing, operations, manufacturing,…) need some idea of when they are likely to take delivery of the software. A team that can accurately (not precisely) give high-level estimates for these things gives their company a real competitive advantage.
So while it is frustrating, there are ways to do it and retain your sanity.