They are not a silver bullet, and there are several realistic ways in which a hardware wallet can fail to protect your Bitcoin. However, it's important to understand that hardware wallets are a high value target and depend on various assumptions holding true to maintain security. Hardware wallets are relatively new, but at least for the time being they have maintained a good track record, unlike the numerous incidents of Bitcoin theft from Internet-connected computers. To date there have been no verifiable incidents of Bitcoins stolen from hardware wallets.
4.8 Swiss Bank in Your Pocket - Hardware wallet.