[f154hfds]

Any vector addition done concurrently will result in indeterminate roundoff errors (to be precise there might be O(N!) different possible outputs depending on the order of calculation), this is literally the point of the field of numerical analysis. But that's really the tip of the iceberg. Beware any mathematical operation done with numbers of vastly different orders of magnitude. Beware spinning your own matmul even if you can't guarantee some normalcy to the numbers in your matrices.

Calculating an average is fairly painless, you can create an upper bound on the possible error that isn't usually dangerous. This isn't the case for all computations.