With extensions you are placing a large amount of trust in the developer and the codebase. For example, the extension requires permission to:
Read and change all your data on the websites you visit
Hence, a malicious developer could not only steal your Steem credentials but possibly even other types of personal content.
I happen to know @yabapmatt is not malicious. However, there is still the possibility that his account gets hacked and a malicious version of the extension is released to the Chrome store. I'm not sure how common this type of attack is and what sort of screening extensions undergo to prevent this.
So in summary, browser extensions can be secure, as if implemented properly they perform all sensitive tasks client-side, which is good, but also can easily leak sensitive data should they be poorly engineered or created/hijacked by an attacker. Please add to my understanding if it's incomplete.