Everything.

Every programming language is an abstraction layer between the programmer and the machine that will run the code. But abstraction isn't free. Generally speaking, the higher the abstraction, the less efficient the program.

C++ optionally provides a much higher level of abstraction than pure C, which makes C++ much nicer to work with. But the trade off is that the program will struggle to run in resource constrained environments, where a program written in C would run just fine.

And to be clear, when I say "low-end hardware", I'm not talking about the atom-based netbook from 2008 you picked up for $15 at a yard sale. It will run C++ based programs just fine. I'm talking about 8- or 16-bit microcontrollers running at <100 MHz with a couple of hundred kB of RAM. Such machines are still common in many embedded applications, and they do not handle C++ applications gracefully.

Compile times get insanely huge.