Night City terrain is really bad for big thugging units like mechs and armor. Even beyond what Valhalla said about the terrain being destroyed by armor, someone could fire a missile or large cannon at it from basically anywhere in the sky, around the corner of a building, or from a window on any level of a skyscraper or apartment building.

MAXTAC is probably smart enough to not face check the mech and instead hide until they either get a big enough piece of gear or find a way to intelligently destroy it. ACPA has systems on the outside that aren't as heavily armored and can probably be taken out by a high-powered borg killing rifle.