For what concerns youtube and vimeo you need to encode with specifications. Using final cut and compressor together with x264 is possible to produce compliant files that are not re-encoded.
But if YouTube are now using the VP9 codec for delivery - your file will be re-encoded anyway? - even if you supply a file within spec?
Or am i confusing the VP9 aspect?
I don't have enough spare time to sit and upload test files! The wife thinks I spend enough of my spare time diving, let know in front of the PC at weekends! LOL