C++ is a huge, complex language. It's probably its greatest downside. Making it even more complex, should only be done with very good reason.
If you doubt this, consider the continued popularity of C, a thoroughly anaemic language by today's standards, with no clear advantages over C++ except for its simplicity, minimalism, and that the language changes very slowly. Well, that and its existing adoption levels. It's not quite the case that every feature C has is also in C++, but it's very close, and I can only name one exception: variable-length arrays (an unpopular addition to C) are not officially supported in C++.
C++'s complexity means that:
* It's very difficult to learn. It's extremely difficult to learn well. It's just about impossible to learn in its entirety. This isn't an exaggeration. (Andrei Alexandrescu might be the closest we have to someone who knows all of C++. He's a world famous C++ expert. Mortals don't stand a chance.)
* Different C++ programmers know (and write in) different subsets of the language. Good C programmers know essentially all of C. (I'll admit I'm weak on C's bitfields, and I couldn't tell you every subtlety of its memory model, but when it comes to C++, there may be areas of the language I've never even heard of.)
* C++ style guides (such as Google's one, or LLVM's one) are long and complex documents, by necessity
* We will never have a fully complete C++ compiler that truly matches the language spec. This undermines the spec; the language you're really using depends on your C++ compiler. To put that another way: portability is harmed because different C++ compilers cannot be relied upon to support the same language features.
* C++ compilers are more prone to arcane bugs, than C compilers
* Different C++ compilers have different arcane bugs, harming portability
* It's far easier to develop tools for C than for C++ (compilers, IDEs, etc)
* There are more C compilers out there than C++ compilers, especially for targets like PIC, or for very obscure platforms, or for particular needs such as safety-critical work. There's even a formally verified C compiler ('CompCert') with near-complete support for C99's features. I doubt there will ever be a formally verified C++ compiler.
* C is easier to mechanically reason about; there are more static-analysis tools for C than for C++
* If C++ were simpler it might have given rise to stable ABIs, the way C has. Instead, even different versions of the same C++ compiler might not be interoperable.
* It's less predictable regarding performance. Template metaprogramming can bloat your binaries for seemingly no reason. In C however, all features of the language map naturally to assembly; the programmer can generally predict roughly what assembly will be generated. This matters to those working with operating systems, graphics, high-performance programming, or where side-channel attacks are a security concern.
C++ is also faster-moving than C, meaning:
* It takes more work to maintain your skills for reading other people's code
* Code can age. Old code looks different from new code, unless it's actively maintained, which means work and risks new bugs. If the language rarely changes, this problem goes away.
* If you're developing a compiler, you'd rather a stable language like C, so that you don't have to make a career out of keeping up with the latest additions to the language. Keeping up with the additions to C++ is more than any one compiler-engineer could hope to do.
There are very few 'conservative' languages like C (there are also Scheme and Forth), but it can be a language's greatest strength. Zig is hoping to be another such language, but we'll have to see if it succeeds.
With all of that said, I tend to favour C++ over C, and I really like pattern-matching. It's something that cannot really be 'faked' with templates or macros. (Another personal favourite feature of mine, named arguments, is similar in that regard.) I think it's rather silly that the major OOP languages have until recently completely ignored this brilliant language feature from the functional programming world, as if it adds nothing over switch/case. Even D, an extremely feature-rich language, still lacks pattern matching.