I use notifications in Thunder and I've had no issues. I haven't compared the difference or anything, but when I've happened to check battery usage it's always been a reasonable amount for how much I've used it that day. It does generate a decent amount of network traffic since it's regularly checking with you instance for it, and that traffic is generated for each account you have reaching out to each instance. That should be how any FOSS app works though, the alternative would be something like Sync where you pay to have actual pushes sent from their server.