If you’re talking about the iOS app, I’d be careful. There are a ton of reuploaders of that app that I don’t even remember who was the original author. It also has you signing in like normal to access your subs. I’ve logged in with my main accounts before and nothing has happened, but you never know.

What's it do?

If you want something you don’t have to reload every two weeks, look for "Vinegar - Tube Cleaner" on the app store. It’s just a Safari extension that replaces the video player on youtube.com with a bare html5 video tag, which prevents all the Google fuckery from loading (including ads).