Advantages:
  Time complexity for hash tables is expected O(1) compared to O(log N) for
  balanced search trees.

Disadvantages:
  The guaranteed time complexity requires a good O(1) hash function.
  Time complexity is only expected time. With any hash function, it is always
  possible for performance to be O(N).
  You cannot easily find the smallest/biggest element of a hash table
  or get the elements in ascending/descending order.

Also see course book chapter 20.6.