It seems like it is “easier” for Sense to detect on and off events if the on-cycles are relatively short (less than 30min) vs. long (4-5hours continuous). Is that really the case ? If so, why ?
It is possible in certain cases, though not as a general law of device modeling 
. Models can sometimes contain a shorter “time-out” parameter than appropriate for your device. It could also happen that, once the algorithm finds an AC “on”, the longer it has to wait for the matching “off”, the more the likelihood that it will encounter noise and falsely associate a different “off” or fail altogether.
We are working on improvements to the detection system and to our device-modeling to better estimate time-outs and juggle hypotheses.
You mention a compressor fan, but I really see two fans in a central air system. The furnace blower that pushes the cool air through the house and the compressor fan that circulates air through the outdoor compressor unit. Which one were you talking about in your article ?
We refer to the compressor fan as just compressor and the air-handler/blower fan as “the fan.” Some air-handlers are 240 V and some are 120 V. Seems like most run in sync with the compressor and some run all the time.
I have also seen device identifications move around based on season. For instance device AC is associated with my downstairs unit during the spring, but the association moved to AC2 come the late summer. Is that typical ?
We have seen this happen with ACs. Some of these issues are due to the general problem of model evolution and associating components: Behind the Scenes: When Good Devices Go Bad - Sense Blog, and some are due to subtle seasonal differences in how ACs and heat-pumps function. We have made some progress in mitigating duplication (especially obvious duplications) and association issues over the past year and continue to work on it. Hopefully, you won’t see the association switch going into the next season.