> I just get a greasy feeling from the lock-in, the heavy marketing, the fact that everyone refuses to speak critically of them unless it’s about anonymous usernames.

There are two things: First, say the Android apk they distribute has a backdoor, and someone realizes that (it's distributed to millions of people, could be that someone checks). Then that's the end of Signal, right? So that's a big risk for them. That's for the "mass surveillance" scenario. Not perfect, but that's something. Second, if you fear a targeted attack, then self-compile Signal. It's not that difficult if you care about it.