Actually posted a similar post several weeks ago. I believe the unbalancing in parks is a big factor, but a huge root cause is Disney's new capacity model. The length of lines is no longer determined by the crowd in the park, but the crowd versus expectations. Disney is going to staff based on expectations, and they do not appear to be nimble enough to make adjustments in time to prevent horrible lines if they are wrong.
For example, if Disney expects a crowd level of 8 and fully staffs park and gets a crowd day of 6, it will feel very light.
If Disney expects a crowd level of 2 and staffs the park at that level and gets a crowd level of 6, it will be overwhelmed and lines will be very bad. Same number of people but massively different feel for the guest.
The new model, IMHO, is if you go on an expected crowded day, you could get better than expected waits, but not worse. If you go on an expected light day, you could get decent lines, but you are risking getting horrible lines. There are some items that cannot have capacity adjusted such as walking paths, parades, and fireworks. If you go on a busy day, these items will be packed no matter Disney Staffing levels.
Therefore the risk reward is the lighter expected crowd, the more likely you will have easier time walking between rides, getting food, spots for parade and fireworks, however you are risking the chance that you will have very, very, very long lines for capacity adjusted attractions. The lighter the expected day, the longer a wait time you could face (in poker terms, the more chips on the table). On the other hand, if you go on a busy day, you could in theory actually have very short lines, but more than likely you will face crowds everywhere (food, parade, fireworks, walkways, etc.). The new ideal time, short term, may be an expected crowd day of 7. It is expected to be crowded enough that Disney will staff it to a high level, however it will not have unbearable crowds. You also could get lucky and have a lighter than expected day and the high staff will make lines very short.
I would say the best way to use current crowd calendars is picking the correct park and a good touring plan. Disney will always be crowded. Knowing that MK will be busy tonight because it is the only night this week they show Wishes is a much more valuable piece of information than choosing a day with a crowd level of 4 over a crowd level of 7.
Finally, long term the crowd calendars will re-calibrate to the new Disney model and fix themselves. They just need the data to make the adjustment and Disney has changed so many variable so quickly it is impossible to build an accurate model. Once this data comes in, the better crowd calendars will be spot on again.