[eluser]rogierb[/eluser]
I don't see how your requirements have anything to do with CI
<i>1. Handle tons of clients and contents (products)</i>
This is mostly Database and webserver related.
If you program poorly in any language, speed will suck. The same applies to CI and PHP
<i>2. E-commerce website automation</i>
<i>3. E-commerce features like cart, mailing system, forums, etc.</i>
Again programming, CI is fast if you program PHP correctly.
<i>There are concerns such as speed, security, and future patches. Or, if I am missing something pleaseadvice.</i>
Any website has those concerns. A very small part of those concerns will be CI.
CI can be as secure and fast as you want it to be. It depends all how well you program.
CI is 'just' a framework that allows you do develop quickly. Out of the box it is very fast and has some nice basic security features.
There are a lot of beautiful libraries out there. On tip I can give for a 'larger' site, go with modules (HMVC). It will make your live much easier.