Hi Moray, limitations of selenium webdriver are as follows:
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No reliable Technical Support from anybody.
 
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It supports Web based applications only.
 
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Difficult to use, takes more time to create Test cases.
 
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Difficult to Setup Test Environment when it compares to Vendor Tools like UFT, RFT, SilkTest etc.
 
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Limited support for Image Testing.
 
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New features may not work properly.
 
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No Test Tool integration for Test Management.
 
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No Built-in Reporting facility.