One of the biggest risks in any CA-based system is with your own private signing key. How do you protect it? You almost certainly don't own a secure computing system with physical access controls, TEMPEST shielding, "air wall" network security, and other protections; you store your private key on a conventional computer. There, it's subject to attack by viruses and other malicious programs. Even if your private key is safe on your computer, is your computer in a locked room, with video surveillance, so that you know no one but you ever uses it? If it's protected by a password, how hard is it to guess that password? If your key is stored on a smart card, how attack-resistant is the card? [Most are very weak.] If it is stored in a truly attack-resistant device, can an infected driving computer get the trustworthy device to sign something you didn't intend to sign?