We have a rather lagerish codebase of C++11/14 and we manage to get people coming from other languages (Java / ObjC) to start working with it just fine. Modern C++ is pretty decent and readable (excluding some quirks of STL naming).

Of course good code reviews and testing is a must to help people to get up to speed and stop shooting themselves in the foot :)

Wow I feel the opposite. Because C++98 and earlier were so deficient in a lot of ways, people who got clever with it often ended up down a rabbit hole of template metaprogramming, boost:: bloat, custom classes for callback stuff, custom classes for reference counting or other smart pointers, etc. etc. Unlearning all that took me some time.

In my 5 years at Google, I've watched many developers start from scratch on our subset of modern C++. There doesn't seem to be a huge challenge with it.