I spent a few hours trying to get a local build of signal-ios working a few weeks ago, in order to write a PR fix a bug with lost voice messages. The xcode project uses a plethora of device entitlements I'm not allowed to have (since I don't have the proper signal signing key). Even after a couple hours of tweaking to get it building and deployed to my device, its currently crashing on startup because it can't access some special signal local device store.

You can certainly get your own build working (without notifications and other features). But personally I found it prohibitively difficult to do so.