The old fire cracker anomaly...hold a fire cracker on its extreme end with just your finger nails and it will explode with little damage...grasp it with an enclosed fist and you run the risk of blowing off your fingers.
The concern is you do little damage and do little to change a large asteroid’s course by exploding it on its surface. With smaller asteroids and comets say small sledding hill size or less, you might melt a good part of it and the ejecta might change its momentum enough to miss earth on the first pass but larger asteroids are going to need something more drastic!
We need a giant Astromatic.
Call Ron Popeil.
It would be a rotating bank of large ginsu-type knives that would slice or dice, resulting in the the now smaller fragments vectoring off their original course and missing the Earth.