Why free SSL are bad for sites?

I am planning to install SSL in my site, and someone suggest me to have free SSL from sslforfree.com, but at the same time some people gave me advice to avoid free ssl, so my questions are
1. whats the differences between free and premium SSL?
2. Is free SSL bad, if yes then why?
Please help me.
I do not have any experience with SSL.

Search engines, browsers or other ranking sites are concerned about SSL, not whether you paid for it or not.
Go ahead and use the free ssl.

Because Google dont trust free SSL.