Yes, true. It is nearly impossible to find a designer which can design _both_ the internal systems workings and the *eye candy*. A strong large project requires a minimum of three good designers.
1. Systems integration designer
2. Low-level designer
3. User Interface designer
Also, the GUI should be easy to replace with out changing the low-level code. Most projects have #2, but few have all three.
Network Manager is not be a good example because a. it is trying to tackle a problem that is close to impossible and that is to cover the needs of all network use cases. When someone comes across a use case that is not supported they say integrate it with NM or too bad. While I support NM as a project necessary to advance Linux, I disagree with this philosophy. b. Most issues I hear about are due to a lack of documentation and issues with drivers.