I think JMM acknowledged a bug where canister was firing with max effect over the max distance. Expect the artillery in the full game and demo v.2 to be quite different, I think.
As for the shells having little effect, don't underestimate the effect of flying metal fragments. A jagged spinning piece of high speed metal the size of a large coin can do enough damage to persuade someone to leave the field.
Some examples, Wenzel Krimer's account, a surgeon with Jagow's bridge under Keist @ Leipzig:
"Our battalion was drawn up in column between two Russian batteries which we were protecting.... a shell came over from ahead of us, exploded instantly, smashed an officer and a sergeant in the chest and head, and broke the legs of twelve men in the column. I fell backwards to the ground, convinced that I had been hit... on close inspection it transpired that a shell fragment had whizzed between my legs and, because its velocity was already much reduced, had got caught in my cloak, pulled me to the ground and stuck in the earth behind me."
It is possible that by 'shell' Krimer means a ball but then he does say 'exploded'. Another example comes from Molostvov, an ADC to Prince Eugen @ Wachau:
"[the french] artillery had a devastating effect, for just as the Prince of Wurttemberg was standing beside a Prussian battlion, a shell landed near him, exploded, and knocked down twenty-five men."
or again, from a french soldier Karl Rohrig:
"we stood in square under a fearful hail ... A shell landed in the middle of the square, its fuse still burning, and dug a crater in the ground. I tucked my head down between my shoulders and waited for whatever happened next. puff! it exploded and did not hit a single man in our company, though the second and third companies suffered heavily. One officer had the top of this head sliced off."
All these gruesome examples are from anthony Brett-James' "Europe against Napoleon: The Leipzig Campaign, 1813, from eyewitness accounts".