While it may have been able to stop some shots, the real purpose of the cuirass was to protect against swords and lances, not bullets. Napoleon specifically ordered his carabiniers to be equipped with cuirasses after seeing the wounds the Austrian lancers were inflicting on his cavalry. No cavalryman wore armor to protect himself against muskets. It was all with melee in mind.
As for lances vs bayonets, why listen to me when you can listen to somebody who was actually there? Let me show you a quote from a cavalryman who was fighting a square on horseback. The quote is from the book "Napoleon's Army" by H.C.B. Rogers, and the quote is the words of a French cavalryman of the 23rd Chasseurs a Cheval by the name of Marbot.
'Of the battle of the Katzback in 1813, however, he has the following story to tell: "The plateau of Jauer and the banks of the Katzbach suddenly became the theatre of a bloody battle, for from every wood Prussian troops were advancing. My regiment... soon found itself in front of an enemy infantry brigade whose muskets had been put out of action by the rain and could not fire a single shot. I tried to break the Prussian square but our horses, hindered by the mud which was up to their hocks, could only advance at a walk, and one knows that without some impetus it is nearly impossible for cavalry to penetrate into the closed ranks of steady and well commanded infantry, presenting a hedge of bayonets. We went so close to them that we spoke with them and struck their muskets with the blades of our sabres. But we could not break into their lines, though this would have been easy if General Sebastiani had not sent the brigade artillery to another place. Our situation and that of the enemy infantry in front of us was becoming really ridiculous, for... our sabres were too short to reach our enemies whose muskets could not fire! Things were in this state for some time when General Maurin, commanding the next brigade to ours, sent to our aid the 6th Regiment of Lancers, and their long weapons, passing over the enemy bayonets, killed in an instant so many Prussians that not only they but also the 23rd and 24th Chasseurs were able to penetrate into the enemy square..." '
This could never happen in our game. Whether that is right or wrong isn't up to me to decide for you, but I know Marbot would find some aspects of this game quite frustrating.