It being closed source doesn't mean that they are any less trustworthy or competent. Trust me, I am totally on board with what you're saying. In this case though, I'd say there would be no need for concern. The developer behind BTT and BST has established trust with their users, great communication and good support.

Also, as for BetterTouchTool being able to replace Karabiner, I have to disagree. I have used both for as long as I can remember and consider them "must haves." But for me at least, they have always been separate in their use and function.

I made the unfortunate mistake of upgrading to Sierra before doing a more thorough check of all my application's compatibility with Sierra. I found out about Karabiner too late - total bummer. I hope an update rolls out soon. (I haven't tried the current "interim" application they created - I need the real thing)