Well, we've seen that active stealth can make a craft disappear from radar altogether in Mac Plus episode 1. In episode 4 however,as they're defolding and preparing to fly into Earth, Yang mentions to Isamu that the probability of their active stealth keeping Earth's defenses from detecting them is 58%. So it seems that the arms race in terms of LO tech vs. sensors seems to move so quickly that stealth tech isn't as effective as it is today IRL.