Problems with Hosted (Outsourced) Email

It is easy to find the benefits of outsourcing email to a hosting company but not so easy to find the disadvantages. Some I can think of are; 1)lack of integration with AD (preventing single sign-on), 2)Internet pipe size and reliability concerns.
Can anyone out there give me the benefit of their experience and the problems they have encountered, or solutions to the ones I have raised.
Thanks in advance.