I have one. It does work, and can distinguish between smoke and CO alarms. You only need one if you have interconnected hardwire alarms; you won’t be able to tell which specific floor it starts from.

A newer solution would be this: https://www.getzooz.com/zooz-zen55-dc-signal-sensor/

No batteries needed, same function, and while I haven’t had any reliability issues with the sound detector I figure the hardwire one is likely more reliable. I just installed one a couple weeks ago, very simple, now I have two detectors.

Does the SofaBaton stuff require Cloud or Internet access to work? Or can it work fully offline? Is the HA integration a cloud or local?

I’m dabbling switching to GrapheneOS. Apple was the lesser of (obvious) evils compared to Google, but that’s becoming less true daily.

I don’t think I can make it my primary device yet, still need to de-cloud a bunch of services, but it’s very likely my next device won’t be an iPhone.