(Element CEO here). Honestly, it depends on your threat profile. Any kind of bridge has to inevitably MITM your conversations in order to work, and we’ve tried to spell that out in all the product info about Element One.
If you want to avoid your E2EE conversations on Signal or WhatsApp being relayed via a service like Element One (because you’re an activist or whatever), then your options are to not bridge at all, or run a bridge yourself. Clientside bridging may be an option in future, but reliably running a bridge in the background on mobile is somewhere between non-trivial and impossible.
Finally, we are currently crunching on getting E2EE to work nicely between the bridges and the Matrix clients (so bridged conversations are stored E2EE on the Element One server) and it should be coming in the coming weeks. It’s worth noting again that even when that lands the bridge will still necessarily be able to see bridged conversations at the point of bridging.
TL;DR: if you don’t trust Element with your Signal conversations for whatever reason, don’t hook up Signal to your Element One account :)