We looked at doing cross-platform, but we would have to retrain a lot of iOS-only or Java-only developers so no luck.
Then the UI engineers often had to create wrappers for the C++ objects (in the iOS Objective-C++ case at least), because ObjC objects only take NSString, while the C++ types only give C++ strings and other such glue things.
Then lazy engineers / deadlines would move business logic into the UI layer wrappers, because development speed wise it was much easier to work with there than to put inside the C++ layer.
Cross platform makes it's own work. But this was with a 'homegrown' C++ solution, so maybe something made by a third party like xamarin might be a very different experience.
One additional issue is that an abstraction layer also adds up in development time. Someone needs to write the platform specific glue, create abstractions that work across all platforms lifecycles, debug if errors happen in the middleware or device firmware...
So it is not always a clear win to not have focused device teams.
Each OS is very complex and no developer can know all of it.
That means no stl or boost, but (and this is not a popular opinion at all) this tends to lead to cleaner code, and more productive programmers.
My experience is that templates do allow for more code reuse than you can typically get in other languages, however they do so at a high complexity and maintenance cost -- not to mention binary bloat and long compile times.
And de-templating, removing stl & boost from 300k+ line projects and debugging it all is something most orgs wont swallow.
I was tempted with robovm, but found the 30s minimum build & run cycle for trivial code changes too slow for me.
And the performance is pretty good for these guys, they're all as fast as objective-c:
https://medium.com/@harrycheung/mobile-app-performance-redux...
At my last job we did the C++ cross platform layer, and it was very painful. C++ is slow to compile and can create bloated binaries. There can be something said for the freedom of having separate platform teams with just a server api.