Building a phone isn't easy (obviously ;)). Been there myself and it may not even be due to incompetence from Purism's side. Your leverage as a small manufacturer with low quantities is just almost non-existing. Their main problem is probably not going to be on the software or hardware design and I think they can probably get a good working dev-kit out in Jun-2018 as they state. Their main challenge is probably going to be on the mechanical design, if they want to mass produce something that is on par with other phones out there quality wise.

I don't know anything about their team and their experience, but that Jan-2019 release date could shift quite a bit. Especially if they need custom tooling or non standard parts. 6 months will evaporate really really fast, when retooling or changes in material is involved.

I love the project though and I hope they succeed. I have been looking for something like this and I will surely buy one, if they can deliver.