This is just a oft-repeated cliche and nothing more. Because C++ is a multi-paradigm language (with admittedly some less than ideal syntax/semantic choices) people overstate its complexity without much study/experience. Herd mentality than takes over and people start parroting and spreading the canard.

For the power and flexibility that C++ gives you, it is worth one's time to get familiar with and learn to use its complexity.

>people overstate its complexity without much study/experience

The is no need in any experience to have ability to estimate C++ complexity. C++ specification is about 1500 pages.