Common mistakes:

• forgetting Time.deltaTime;
• diagonal movement being faster;
• mixing direct Transform movement with dynamic Rigidbody physics;
• applying forces in Update instead of FixedUpdate;
• reading one-frame input only in FixedUpdate;
• applying gravity twice;
• forgetting Rigidbody rotation constraints.

Clamp or normalize diagonal input:

direction =
    Vector3.ClampMagnitude(
        direction,
        1f
    );