I'm sure that physics supports better aim with a more vertical path of the rod, for one thing the path of the hand is aligned with the eye and where the hand goes the rod follows. When I take the overhead cert test I will be using a vertical stroke for accuracy tests for sure.
But as an avid sight fisherman I rarely cast with a vertical rod position because it's harder to get as fast of a load into the rod for me with the upright angle in time to make the connection with the cruising fish, not to mention potential collisions with stainless hvy hooks and weighted eyes to right the hook on very shallow flats, etc.
By harder to get a load I mean that when walking around in the water, holding the line in the hands or when casting after one shot to take another, I find it easier and more natural to throw the necessary backcast with an outward angle to get things going to get enough momentum going to put the fly where it needs to be. After years of sight fishing I have become 'accurate enough' with a 45 degree cast and have avoided the vertical position for fishing saltwater.
Accuracy with spey casting is another topic entirely. To use a very vertical path you usually need to set the anchor closer, thus you would use a shorter line and probably more of a Scandinavian technique. For most of my fishing I prefer a longer line and traditional technique, and I am finding that you can get pretty accurate coming off the anchor placed a rod length away and to the front just by paying close attention to the angle of aim and having a smooth stroke to the correct release point for the target.
When two-handed overhead casting it's quite a bit easier to use a (near) vertical path of the rod and you can create a tremendous amount of power for accuracy at distance with a little practice.