It's the opposite, much easier making a mistake on a paper form that doesn't direct you. The online form removes things that doesn't apply to you based off of your age, if you don't have children etc, it also tells you if you haven't been an LPR long enough to file etc (while with the paper form you can still send it in not knowing you're not eligible yet, and your petition fee is in the hands of the lockbox people - will they accidentally accept your filing or not? Will they cash your check or not? You get denied later on, you don't get your money back). And if you happen to misspell something or not get something right (same with the paper form, people make mistakes on those all the time), the IO manually corrects that at the interview. The online filing is several times better and safer than paper filing.