Typically it requires more time - and training/experience - to write automated test cases and perform overall planning and setup. And you still have to keep the test execution in mind because the tests should be designed so that they would be executable for a tool you will choose for this purpose.

The main problem, IMHO, is that test automation platforms typically bring new challenges so one should analyze all pros and cons after evaluating a number of tools, and then, having picked one, proceed with test design.