It's my understanding that SteemConnect is just as trustworthy as your keychain. The point of it being a website is that it's accessible across all devices and operating systems. My understanding of SteemConnect is that they never see your private key. They use your key to create a permission token on your device.
The website simply called the Metamask browser extension to sign and broadcast the transactions for it.
And what happens if the contract is meant to steal your money? We can't really vet any of those transactions that pop up for legitimacy. We just trust that they do what the website told us they would do.
When it really comes down to it one has to trust the code. We expect that if the code is malicious a white hat will whistle-blow on it.
I would really love to be corrected about SteemConnect or why this service provides more security. In the link above I concluded that a browser extension would be a great way to provide the illusion of security...
However, why did you make your own product when you could have just extended SteemConnect into a browser extension? It's all open-source.