Yes, hitbox is tournament legal. It simply replaces a joystick with directional buttons. However, the question is completely valid.
The major concern with hitbox-style controllers is SOCD cleaning. SOCD stands for simultaneous opposing cardinal directions. An SOCD cleaner prevents two directions from being registered simultaneously.
Consider holding back and forward at the same time when playing Street Fighter. If a hitbox does not have an SOCD cleaner, both back and forward would be registered as inputs, potentially allowing you to block both directions at the same time, which is not meant to happen.
Fortunately, most if not all hitbox-style controllers have SOCD cleaning. If you are still unsure about which hitbox to buy, check out our top hitboxes list.