Crashing into a body requires affecting it's path further in advance because we only have such big rockets that can only add so much dV.

Energy density of a nuke is way higher than rocket fuel of equivalent mass.

Precisely timing a device is trivial in the grand scheme of thrust vector control, orbital navigation, and all of the other control systems operating the device. Like sure it's hard, but an extra PCB, sensor system, and software isn't going to exactly break the bank.