Thank you for have replied to me!
I was not aware of this security issue, thank you for let me know.
For now I AM using a Comodo certificate and it's very important to know this issue in Let's Encrypt.
There's no reason not to use LetsEncrypt. There are no security issues with letsencrypt. The posted link mentions that LetsEncrypt gives free SSL certificate to every website owner in the world as long as you can prove you own the domainname, which is exactly what they advertised.
Comodo domain validated SSL certificates are not free and it is not better than LetsEncrypts domain validated SSL certificates for website owners in any way.