Question: Cambridge Dictionary gives this definition about "complain"

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> to say that something is wrong or not satisfactory
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and this definition about "complaint"

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> a statement that something is wrong or not satisfactory
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I guess both of them means the same thing. However, [Google Ngram](https://books.google.com/ngrams/graph?content=to%20complain%20about%2C%20make%20a%20complaint%20about%20&case_insensitive=on&year_start=1800&year_end=2008&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cto%20complain%20about%3B%2Cc0%3B.t1%3B%2Cmake%20a%20complaint%20about%3B%2Cc0#t1%3B%2Cto%20complain%20about%3B%2Cc0%3B.t1%3B%2Cmake%20a%20complaint%20about%3B%2Cc0) shows that the former is much more commonly used that the latter

[![enter image description here](https://i.stack.imgur.com/IyIrh.png)](https://i.stack.imgur.com/IyIrh.png)

Are those two expressions interchangeable?

Answer: 
When in doubt, I find it helpful to simplify the sentence. Consider these: 

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> The conclusion is uncertain.  
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>  The conclusion is final.  
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>  The conclusion is X.
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Clearly, whatever the conclusion is, it's singular and needs a singular verb. 

Now let's look at what the actual conclusion is:

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> Both are harmful.
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Again, clearly, "both" refers to two things and thus requires a plural verb, "are". You could check that by replacing both:

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> Cigarettes and gun battles are harmful.  
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>  Angry dragons and mean dogs are harmful.
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Now, let's take that last *conclusion* example -- *The conclusion is X.* -- and replace X with what it actually is:

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> The conclusion is [both are harmful]. 
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When you look at it that way, it becomes clear that the sentence as originally written is indeed correct.

My conclusion **is** that your friends **are** wrong.
In relative , noun or adverbial clauses we often face these kinds of sentences. E.g “Who I am is not important.