When I’m working on a project - I find a designer who is focused on the task I need done. UI designers for UI work, logo designers for logo work. Here is why.
Designers who specify do a better job.
Think about this - where would you rather get an Italian sandwich? The super market deli that sells an Italian sub, or a local shop owned by Italian's who have been making sandwiches for years with ingredients imported from Italy.
The same concept applies to designers. Sure, some designers are good at a broad range of things. However, if you want a really good design - work with a designer who specializes in their field.
A designer who specializes in a type of design will know the nuances and best practices much more than a designer who produced 2 logos, 4 landing pages, 6 web apps.