If you consider how thick a brake drum is on the face and how much thicker the same area is on a disc brake you will notice that installing disc brakes moves the wheel and tire out up to five eighths of an inch on each side of the car. This increases the front track as much as an inch and an eighth, and causes the tires to rub against the fenders.
You can buy a kit with wheel hubs that move the mounting flange inboard to counter this affect but it adds a custom wheel hub and new bearings to the price of the kit. The cheaper kits have you reusing the drum brake wheel hubs and don't mention the tire rubbing issue instead directing your attention to the pretty zinc wash (that rubs off inside of two hundred miles or the drilled rotors (that promote cracking) or the slots that just add cost (though they do help remove any water that gets between the puck and the disc face).
Your drum brakes can lock up your wheels at any speed at least once. Were disc brakes are required is in a racing application; say Watkins Glenn, or Riverside, or Sebring, where you are on and off the brakes continuously for twelve hours. Disc brakes resist fading by dissipating heat better than drum brakes. On the street unless you live in the mountains of the Colorado Rockies (which are better described as the Colorado Gravely Hills) or the hard granite rock of the North Carolina Smokey Mountains you won't need disc brakes. If you spend most of your time climbing up and down mountain grades or dragging the brakes crawling around on the interstate by-pass of Atlanta or Los Angles your drum brakes will work just fine.
The factory went to disc brakes not because they can stop a car any faster than a drum brake (still retained on all Semi Trucks because the drums actually do work better with air brakes) but because they are cheaper. Those backing plates, drums, springs, washers, pins, retaining clips, and self adjusters all add up to more parts and labor cost to assemble than just bolting on a caliper and a rotor. You will notice that drums are still used on the back of all but sports cars even today as the emergency brake requires a hand activated lever: and disc brakes require three times the effort to lock the rotor than a drum does (drum brakes are self activating which is why they are used on trucks, and why those springs are so strong, because it is the spring that pulls the shoe away from the drum where it wants to wedge itself in place).