Is there any "difficulties" involved for an H1 transfer from outside US

I have a situation here, where I am currently working for Company A, with whom I have a valid H1B petition and visa stamp till Sept 20th 2014.

Company A is not ready to send me to US. So because of this reason, I found another company B who is willing to transfer this visa.

I know that once transfer is completed, I would be able to travel with new 1-797 petition with Company A h1b stamp ( within Sept deadline).

But actual questions are following:-

  1. According to many sources, H1 transfer outside status (without US payslips) is difficult. Why? Is there any risk involved in such transfers comparing to one done in the US itself. I have n’t seen any post asking this query.

  2. If I am going for Company B. Then i need to resign now (July) & serve notice period for 2 months. So best time for me travel would be Sept only. But visa stamp expires on 20th Sept. Is it ok for me to travel.

Hoping someone be kind enough to answer my above queries.

Thanks a million in advance.