The bridges are all open source, and they use matrix synapse as their server installation - though their client is a closed source fork of element with changes. You can use any matrix client to connect to it, and they say it's a standard synapse setup.

If privacy is a concern, bringing your own client should remove that concern as the rest is open source. It's also e2e encrypted, as any matrix server is.

I self host my own matrix homeserver with bridges set up using their code. The only bit of their stack I can't use is the client. I don't like that that's closed source, that's frustrating.

Edit: while writing this two more people made the same comment. Sorry!