Would you rather Unit Test or System Test a Cloud Based Application?

A cloud application often has a number of difficult verification areas, such as elasticity, load test,stress test etc.

Because there is so much interaction with live cloud whose response can change between every test execution, according to me unit tests would be quite futile.

System tests seem to be the better option amongst the two.

I would like to know if anyone has idea of a legit situation where unit tests can be used instead.