There are two forms of optimal linear attacks according to the human body biomechanics. The first is the front kick and the second is the drop punch. The first comes in a straight line in the center of the human body to the pelvis area. The second comes in a straight line to the human body head area. Both lines are sort of parallel to the ground somewhat but use a slightly upward diagonal motion across these lines(center body/gravity pelvis area, and the top of the body/the head).
While each method gives protection from being subject to the other form of attack simultaneously, there is the element of reaction time. In this case it is the time that takes to lunge pivot the torso slightly to center the kicking leg and lift the knee up to 90 degrees vs. the time it takes to extend the arm for a punch(the elbow is naturally already positioned closer to the proper punching motion). The equation in this case is over the longer distance there it is faster to use one lunge kick to the opponent's pelvis area than to use two lunges and drop punch.
Positioning of the torso, the natural reach of each limb, and the reaction time play a role in the ability to counter kicks with punches and vice versa.
When you lunge for a Drop Punch you land on your front foot with torso tilted forward and shoulders twisted at the moment of contact to ensure maximum weight shift and maximum reach. The front hand is stretched out to pass your opponent's chin and retracted before you complete the landing.
When you lunge with a front kick, you land on the rear foot front leg extended to reach your opponents underbody in the groin area or if he twisted his body off center to tackle his liver from underneath. Your torso is leaning backwards at the moment of impact and during impact you retract to maintain optimal impact.
Aside from various body types, such as shorter legs and longer arms there is a play of the time it takes to lunge and kick and the time it takes to lunge and punch from the shorter distance and the longer distance. It takes less to lunge and kick from the longer range than the time it takes to lunge and punch from the longer range. At the same token it takes less to lunge and punch from the shorter range than it takes to lunge and kick.
The apparent consideration to take is when you lunge and punch, your reach to the opponent's head is optimal and at ease. At the same time your pelvis is behind and twisted.
When you lunge for a front kick your torso is tilted backward so your reach to your opponent's pelvis is optimal, and your opponent's reach to your face is none!
I suggest at some point in your training if you have all the time in the world to try a drill to test your capabilities as follows:
Both training partner are in the attacker role and start attacking each other upon the instructors command.
Drill 1. Opponent A executes lunging and front kicks from the long range.
Opponent B tries to straight punch the attacker at the same time the attacker is kicking.
Drill 2. Opponent A executes lunging and drop punches from the Short range.
Opponent B is executing a front kick from the short range at the same time.
Drill 3. Attacker is executing a punch from the long range
Defender is executing a block kick to the opponent's thigh or hip(blocking foot is positioned diagonally on the limb so it does not slip).
Drill 4. Attacker is executing a kick from the short range.
Defender is moving lunging with a short drop punch pivoting his torso away from the kick.
Note: These drills should be done carefully starting slowly with increasing speed and minimum penetration into the target with protective gear.