This is a difficult question to answer.
The answer depends upon 1) how much knowledge you have, 2) what type of subject you are writing about, 3) how picky your client is, and, 4) if you are writing just to get keywords on a page or if you are writing to attract links and earn high rankings.
There are many different types of topics. Here are just a few....
A) thoughts and opinions (should the USA sell natural gas to Ukraine?)... You have to know a lot about many subjects to write that type of content, and then you must analyze it and explain it. It is really easy to be wrong and look like an idiot. It is really hard to write this without doing a lot of research and knowing what and where to research requires experience.
B) simple reporting (like news reports)... This usually isn't difficult writing as long as you have all of the facts
C) technical and factual (like explaining atomic absorption flame spectrophotometry). If you try to write this without a very good knowledge of chemistry and physics and familiarity with the language of those disciplines you will sound like a complete idiot starting in the first sentence. Any knowledgeable reader will know that you are not prepared to write that content.
I spend most of my time writing content in areas like A and C above. I am not writing just to get keywords onto a page. I am writing to produce pages that are best-on-the-web for their topic area.
I earned several college degrees in my subject area and have about 30 years of experience working and teaching in the subject area. With all of that background I still have to do a lot of research before writing. Sometimes accurate information is very hard to find and then explaining it clearly is still difficult. Knowing what people want to know is easy but knowing what they really need to know requires experience. Being able to speak like an educated person about these subjects requires immersion in an industry or academic community.
Now, I want to spend my time writing about a topic that is very close to my education and work experience. But, because I want to write competitive content, some of the best that is on the web for the subject areas, I am going back to school for a few years to learn more, to become conversant in the language of a new subject, and to understand what people need to know instead of just what they want to know. That is the valuable information.
All of this is not needed if you are writing about common events that do not require great background knowledge and communication to expert or technical audiences. There are a lot of websites that don't need such background. But there are many who must have it.
Don't try to bullshit the readers. The important readers will smell it immediately.
If you feel that you are not qualified to write the content for certain clients, I would not hesitate to decline the work and instead spend time writing for clients where your knowledge and background are strong - you will be more productive there and produce a better product.