Shaw's Arms and the Man is not realistic, if we define realism as characterized by an attempt to accurately reflect life as it really is. I would agree that is an example of nineteenth century piece bien faite, using an improbable, if charming, romance plot to criticize unrealistic ideas about love and war.
From the beginning, Shaw takes threatening situations and casts them in the least frightening of lights. A deserting solider, Bluntschli, climbs a balcony to hide in a young woman's room. Realistically, this would be very threatening to a young woman, but Shaw casts it in the lightest of tones. The two share a cheerful conversation, full of banter. They begin the process of falling in love.
The play depicts the pitfalls of falling in love with an ideal or concept—a false persona. Raina falls in love with the idea of Sergius as a dashing war hero, and he does the same with her, as Raina is of a suitable class for a man like him. Nevertheless, he prefers Raina's servant, Louka. In a realistic play, this might cause jealousy and heartache, and it would be extremely unlikely that a servant would end up with a man from Sergius's class. In this play, however, that is exactly what happens, and the timely appearance of Bluntschli diverts Raina so that she is not even superficially wounded by Sergius's betrayal.
Likewise, war is criticized for its incompetent execution, false heroics, and waste, but we see nothing of the real agony of warfare. Bluntschli carries chocolate bullets and that, from the start, characterizes the lighthearted way fighting will be depicted in the play.
Shaw uses a comic vehicle to make a point about war and false ideals, delivering his message with the proverbial spoonful of sugar.