Determine object brightness/"detectability"

[Shimonu]

I had a bit of a surprise tonight. In my last session I took a 120s exposure of the Soul Nebula. I try to get a quick shot of something at the end of my sessions as a teaser for next time. But stretching this sub I can't even see a hint of the nebulosity, I used plate solving to find it and I've confirmed with astrometry that it's in the frame. My understanding was that surface brightness was a good indicator of how bright an object is i.e rough estimate of if you need a longer or shorter exposure compared to something else. But that seems to have gone out the window now as I have 30s subs of Eastern veil which gives signal and that's at 18.15 mag/arcmin^2 while the Soul Nebula is 12.74 according to Stellarium.

So is there a good way of determining how easy or difficult something is going to be to photograph?