this breaks e2e encryption:

It’s also worth noting that end-to-end encryption is necessarily broken as messages to (and from) WhatsApp, Signal and Telegram pass across the bridge(s). The bridge(s) operates in Element’s trusted EMS environment, with no content scanning or datamining, but currently bridged conversations are not stored end-to-end encrypted in Matrix (they will be in the future).

and per the Element CEO, setting up these bridges necessarily MITMs your conversations. Element is incorporated in the US and that means it's subject to National Security Letters and other kinds of sealed warrants -- you shouldn't trust them or anyone else to run a bridge.

warn your contacts not to set this up -- they can accidentally MITM your conversations with them if they set this up without realizing what they're doing and you'll have no way of knowing that it's happened. hopefully, Signal decides to treat this as a vulnerability and blocks the bridges from its network so people don't get inadvertently bitten.