If security and privacy are of absolute priority to you (journalist, activist) then using a OS with closed source essentials is absolutely absurd.

If security and privacy are of some importance to you then using a OS with closed source essentials due to ease of use or development environment only increases the number of years until a strong privacy easy to use OSS system gets rolled out more widely. Your actions have broader impact than just your priorities.

Finally, there is more than just the software. Getting closer to the metal you have to contemplate bootloaders trust and verifiability.