Non-Attorney opinion:
Of course some attorneys would say that. Some attorneys prey on unfounded fear........and some make stuff up sometimes. My wife and I did the visa interview process, the I-751 , and her N-400 (5 year rule) processes ourselves with no issues. The N-400 was was the easiest. It was a guided online process of answering questions and uploading a few documents (5 year rule). Even with an attorney, YOU still have to gather the evidence and documentation.
A denied N-400, in itself, would not cause an NTA. For an attorney to state that as a blanket statement is malpractice, imho. That might happen only in an extremely rare case where the legal residence was fraudulently acquired. I don't recall seeing any of those.