The Tor Browser is scrutinized heavily. I know that is kind of a fallacious argument, but they have a routine presence at DefCon and they are really committed to protecting people around the world. At DefCon last year, the Tor project talked about the biggest security concern as countries who monitor the entry points to the network, and the challenges with getting those IPs distributed confidentially and keeping them hidden. (If you don't already know, Tor does require an entry IP address list before the anonymization occurs: countries can arrest people who visit these URLs, so this is the big challenge right now.)
Someone who knows more can probably elaborate, but after hearing them present year after year, and how much global advocacy they engage in, and their transparency, I find it unlikely they have NSA spooks embedded.
I use it when I need to read something objectionable through a VPN. (Still not perfect, because I run a VPN on EC2 and the exit zone is in the US...)
Not trying to be flame-baity here, but with Trump's ranting about making ANTIFA a terror org, and with the recent legislation that allows warrant-less IP tracking, I am legitimately concerned I might end up on a watch list because I visit a website this admin finds objectionable.