So, the issue with the String class isn't with the String class itself - it's a very useful class (in the right situations). The issue is with dynamic allocated memory causing fragmentation in the heap, which overflows and crashes into the stack.
Part of the problem IS with the String class itself. When you want to append one character to an existing String, the length of the new String is the length of the old String plus the length of the String to append. That amount of space is allocated, and the old String is copied there, then the String to be appended is tacked on, then the old String's space is freed.
Append another character, and the whole process is repeated, because the String has no room to grow.
On other platforms, extra space is allocated, so that there is some room to grow. Perhaps 10 extra bytes, so you can add 10 characters, before a malloc/copy/free operation is required again.