From my experience i can say the following are as drawback
1. Effort to automate large number of test cases
2. It may take too long time to automate a entire project. It will take
long time to see a return in investment
3.The Tool provider will say our tool can be used eventhough you made change in UI. But it is not applicable to all types of changes. On we changed any UI we should put effort to automate it
4.If you want to test your application in muti browser, you should confirm with some trial versions.
5.Also if your application takes time to load any object, the tool script should be written to wait. The time we cannot be determined
You are making some good points here, however they all can be solved. Our tool, SymbioTeam (SymbioWare Inc.) offers a full spectrum of Web test automation, and our custoemers love it. They routinely run hundreds of thousands of automated test cases per year for their most business-critical Web-based applications, and today cannot even imagine their life without it.