My I-140 NIW has been approved in Nebraska Service Center without lawyer service. I also replied REF (LC waiver and past record) by myself. My experience prove that you can file I-140 yourself, save money, get the case approved that will be denied if prepared by the lawyer.
If you DIY file I-140 NIW too, email me at ysh_69@yahoo.com. I am happy to answer your question free.