I believ "Not all designers are created equal". For instance, I have a very deep understanding of SEO, some dont. Some may code much better than I do, some are better at illustrations, some are better at web and some are better at iOS designing.

Simply check their portfolio and see if you "match" his or her style. If you are into minimalism you dont want to work with a designer that really likes to put tons of fancy effects on their design.