For the vast majority of members, booking is a piece of cake. They get the dates and the accommodations they want with no problems or issues.
For others, any and every part of the booking process can turn into a nightmare. A lot depends on how far in advance you book, what dates you are trying to get, and sometimes what type of accommodation you are looking for.
Our home resorts are BLT, AKV, and Aulani and we almost always book as soon as the 11 month window opens, and we have never had difficulty getting exactly what we want. However, if I were to wait until August to book a 10-day trip for November for a BLT Standard View or Theme Park View, or Aulani Standard View, or AKV Value, I would probably end up with a patchwork of split stays, some in accommodations I didn't really want and/or at resorts that weren't my second or even third choice.
Like supersnoop says, the earlier you can plan helps, and staying flexible helps, too.