[BUG] QSVEnc doesn't works

[raven-au]

But having said that isn't it more or less impossible to guess these?

Guessing them intelligently: yes
Guessing them: No. For min/max Luma a fallback might be 1000/400.

I seem to recall that not setting the luminance correctly on an encode made a noticable difference.

It does! That is the point. Some players might guess some values in case they are not present, but whether the output looks like it was intended by the one who originally mastered the content is a totally different thing.

-> make sure to set all your content signaling settings properly, just because device X shows something HDR like, that doesn't mean that:
a. other devices will show the same
b. you did a proper job

I worked out these settings where needed right from the first HDR movie I encoded because things wouldn't trigger HDR or if they did they would often look off,

What I do now is look at the movie metadata and set them according to that.
But, with your comment above I'm now wondering if I should set a luma fallback when it isn't specified in the metadata?

Yeah, I'm working out what I need to do to produce a proper encode now,   

Ian