When it first started getting popular I was curious about whether they were trustworthy, so I chatted with the devs and they said it was 100% open source. I looked through the source and found a pre-compiled binary blob buried deep in the folder structure, and they came up with some strange excuse that I can't recall and then it disappeared after a while.

Also, the evidence that it's not end to end encrypted by default for many operations is out in the open.