Hello Delusions of Grandeur,
the fact they are the only toughness based army in the game. The Tyranids are a close combat army. Ya, there are shooting elements, but they’re only there to crack open the shells of vehicles to get at the delicious bio-nugget inside, or to disrupt the opponent. You will not win a shooting war against anyone. To achieve victory you have to get in the opponent's face and bring the fight to them. They boast some of the highest initiative creatures throughout their army, but lack in grenades. They can field large amounts of cheap troops and can still have massive monstrous creatures bearing down on you. The Tyranids are a dance on the battle field. They have many abilities that require you to be within radius of other creatures, and you have to judge if charging into units behind cover is worth it at the time.