The three design types: system designers, interactive designers, and visual designers, can be treated as a hierarchy. System design mainly feeds into interface design, and interface design mainly feeds into visual design. It is more important to execute successfully at the system design stage than the other two, because decisions (both good and bad) made at that level cascade to the others. It is hard, if not impossible, to make up for shortcomings in system design with an amazing interface or visual design.
The question that is often faced is how much focus to give each of these types of design, especially given their hierarchy. Do they all get roughly equal treatment, or some other division? The answer basically boils down to how much cognitive friction (how much an interface “gets in the way” and slows you down) can your customers be expected to tolerate.
If you are designing a system for a users who will likely find value from a poor user interface and do not care much about how their tools look and feel, then you probably don't have to spend as much time on interface and visuals. But if your system (perhaps a game) needs to wow the crowd, then you may want to make a thorough investment in all three design types.
Learn how Digitally Inclined Solutions can help you navigate through the design phase.