Drawback of View State.

The _VIEWSTATE hidden form field can become very large. Stuffing too much data into View State can slow down the rendering of a page because the contents of the hidden field must be pushed back and forth between the web server and web browser.

Because the view state is packed with the page, it inevitably adds a few extra kilobytes of data to the payload. A complex, real – world page – especially one that does not optimize and restrict the use of the view state – can easily find 10KB of extra stuff packed in the HTML code sent to the browser.

Since it is passed as plain text to the client. Anybody can tamper this value, because of this you shouldn’t store any important data in the view state.