Sampada in response to your questions...

Risks 

1. You might end up NOT getting H-1 visa due to limited slots allocated each year.

2. You might not get placed in a job for a looooong time. That means no income. I know people who were on the bench for more than 6 months.

3. If you do not have a computer background ....you will have to study more than you bargained for. This is not necessarily a risk but something to keep in mind.

4. You have to develop excellent lying skills.

If you have to absolutely resort to these consultancies....

like i mentioned earlier do some research and make sure you go to one that has been around for a while ....has a good marketing strategy and has a good client base. If you are not open to relocation, make sure you let the consultancy firm know this in advance. Don't worry about signing their bs documents....they can't sue you. And, if you are not computer savvy....start preparing.