Long long is just a data type. It doesn't matter what data type you use. Only reason it would ever cause a problem is if it's implementation defined (long long may be, I can't remember) you could run into issues if you port it to a new compiler.
long long is a C/C++ fundamental type. So I did not see any problem except that you will try to compile the program with using an old compiler that does not satisfies the current C/C++ standards. In C long long is used since 1999 year.