This is true, even if you were given brief instructions on how to perform the surgery.
When it comes to medical procedures, you cannot be confident you will not make a mistake, unless you are a doctor. You could even potentially make a big mistake you do not know about which could end up harming you later.
If you are at all wise, you would decline to perform surgery on yourself.
Trying to do your own estate planning is not entirely dissimilar.
You can make mistakes there too, as Market Watch discusses in "The problems with doing your own estate planning."
Of course, in estate planning you are not going to have any health problems, if you make mistakes. Making a mistake in an estate plan will not kill you.
However, if you purchase online estate planning documents and fill them out based on brief instructions, you could hurt your family.
You may never know what mistakes you made.
Your family might never find out about any mistakes, until after you pass away. However, the mistakes can be very costly.
Before attempting to create your own will by purchasing a form, ask yourself how confident you are that you will not make a mistake.
Keep in mind that you do not know what you do not know.
Reference: Market Watch (August 16, 2017) "The problems with doing your own estate planning."