They actually had security audit done by Trail of bits. Here’s a link to their reddit post

First time I hear about it as well. As far as I know Simplex doesn’t use phone numbers (or any identifiers for that matter). It can be alternative for signal but it will not help if you need SMS app.

There’s also app for desktop PC called freetube. It’s similar to Newpipe, it lets you keep your subscription list offline without any account and if I recall correctly you can set it to use invidious instance as source. Unlike newpipe it doesn’t support peertube.