System architecture can vary a great deal from engineer to engineer, but when working in the mm Wave frequencies it is almost guaranteed that a down and/or up converter is involved. Mixers come in several different flavors at these frequencies (subharmonic, harmonic, biased, single balanced, and double balanced to name a few). Each presents its own advantages and challenges. Many issues are straight forward, such as filtering out the LO on an up converter, but many issues can hid in the details. For example, low IF for a single down conversion can lead to in band spurs that will pollute your signals.
Each system must be developed with the end goals in mind and a eye towards the future.