The one useful feature here might be automatic backups. Losing chat data is the biggest downside for the friends who didn't like signal. It's not worth the risk for them.
I have automatic backups set up with foldersync, and it works pretty well but feels janky to set up. I also have to periodically clear the files from my Google Drive trash folder else it eats up my storage.
Signal on the Google play store isn't FOSS but it facilitates the delivery of notifications to the user. The protocol itself is open source and notifications are handled securely even with Google play services on the phone. If you want a FOSS version, use the APK on their website, it has an auto updater.
It definitely isn't energy efficient. I installed Signal as an SMS app out of curiosity when I first degoogled but the battery drain was insanely high and the highest of all my apps.