Why should I trust SuperSU / TWRP / Clockwork / ...? One more phone, same questions. Despite being a developer myself, I am quite a bit far from Android development. However I prefer having something from SlimRoms rather than stock bloatware. But the question arises:

Is there any sensible reason to trust my credentials and privacy to popular software like SuperSU, TWRP, ClockworkMod, custom CyanogenMod builds and so on?

I am also aware that TWRP is open source. Though I have no clue how to build it, and who did it for me with which modifications.

Sorry if the question sounds a bit unethical for somebody.
The same reason that people trust already-compiled builds of Ubuntu, LibreOffice, Linux kernels and mainstream-compiled builds of thousands of other open-source efforts. 

Namely that they are being backed by large organisations of people, the code or patch file used to build it is required to be submitted (and is most likely diff'ed against the original build, making the insertion of malicious code extremely hard to hide), and the maintainer is usually a trusted member of the community around said program.