Sure, I can go around and tell everyone I know that they shouldn't bridge Signal to Element One because <cryptonerd rant>, but there's no way for me to tell that my advice has been followed. Previously, the chances that someone had set up such a bridge were small, but by nature of making these things accessible, Element One also increases the risk of my conversations being (benevolently?) MITM'd.
I think it's a great offering, but I share GP's reticence about a system that puts what used to be zero-knowledge conversations in plaintext on a third-party server...all without me realizing.