The ATACMS is not a ballistic missile itself, because its trajectory does not follow faithfully like a ballistic form. Like PrSM, the terminal flight profile is believed to include a near-vertical dive toward its target to maximize terminal speed and accuracy, terminal speed is likely greater than Mach 3:
The apogee of a ballistic missile with a range of 300 km is normally above 80 km, and can reach an altitude of 100 km. ATACMS only climbs up to 65 km in altitude, allowing the missile to avoid most surface-to-air defense systems while maintaining the accuracy needed for precision strikes.
This is why the Russians are unable to respond to HIMARS/M270 fire with ATACMS, the Russian ISTAR capability in depth(>250 km) which basically depends on the VKS is still too small to be able to track and locate to carry out counterbattery fire against platforms launching ATACMS. The aeroballistic trajectory of the ATACMS complicates the entire Russian counterbattery effort, combined with the post-fire movement of the ATACMS launch platform makes everything more difficult for the Russian ISTAR.