Issues with estimating, particularly something like this, mostly comes down to meta-ignorance: you don't know what you don't know.
Given a particular problem of a similar sort to something he's worked on, an experienced engineer will be able to list 10 different issues that might crop up.
An inexperienced engineer won't know of those potential problems and will just see the most optimistic path possible.
When you're moving massively outside your familiar zone, many unexpected problems will crop up. Best solution is a phased approach. Pick the bits you judge as most risky (you should hopefully have some idea) and implement prototypes in those areas, in the hope that most of the scaries will crop up. You've then got a better idea as you move on.
I've written this in the context of doing engineering, but it's equally true in all fields - like trying to sell a product into a new an unfamiliar market.
Given a particular problem of a similar sort to something he's worked on, an experienced engineer will be able to list 10 different issues that might crop up.
An inexperienced engineer won't know of those potential problems and will just see the most optimistic path possible.
When you're moving massively outside your familiar zone, many unexpected problems will crop up. Best solution is a phased approach. Pick the bits you judge as most risky (you should hopefully have some idea) and implement prototypes in those areas, in the hope that most of the scaries will crop up. You've then got a better idea as you move on.
I've written this in the context of doing engineering, but it's equally true in all fields - like trying to sell a product into a new an unfamiliar market.