In padel the ball bounces first, then you strike it below hip height into the diagonal box. In pickleball the rule is: below the waist, paddle head under your wrist.
The underhand serve is not a handicap, it is a design decision: it takes dominance away from the serve and makes the rally the actual game. In padel you let the ball bounce once and strike it below hip height into the diagonal box. In pickleball the ball is struck below the waist with the paddle head staying under your wrist, and you serve from behind the baseline diagonally past the kitchen line; alternatively the rules allow a drop serve, where you release the ball and hit it after the bounce. The typical mistake is the same in both sports: full throttle. An ace is not a realistic target here — a safe, well-placed serve that simply opens the rally does far more for you across a match than any high-risk attempt.