It isn’t an artificial horizon the way a reflective, liquid surface is. Those are quite accurate. As for gravity, it can have a small effect on any tool or device that tries to measure ‘down’, it isn’t much, but when you’re talking about extremely precise things, the effect is there. That said, I got an A-12 a few days ago and have found what I think is the culprit behind “… the bubble-octant was the worst option for attempting celestial sites while navigating on the land before GPS was invented.”
The bubble is dancing about like mad and it isn’t small, making extreme accuracy difficult. I’m going to need a lot of practice on the ground before getting in a plane with this thing!
As far as being innacurate though, it looks like most of the inaccuracies come down to using the thing correctly. I did find this: https://journals.lib.unb.ca/index.php/ihr/article/download/27852/1882520608/1882520854
Sights can be taken at any time, and even through a hazy or cloudy sky without a horizon, which is a nice advantage! All you need to be able to see is the object you’re shooting.