Active Defenses are some of the most important Upgrades in the game, because their use is baked into the game's combat math. That is to say, it is assumed you will buy one in the same way you buy Weapons, and without one you are going to have trouble keeping up with others.
When put this way I wonder: Should there have been Default Active Defenses the same way the game has Default Weapons? Probably not, if the concern is that starting characters are too vulnerable without them, might as well just give those a proper Defense boost.
Active Defenses generally give between 3 and 5 Defense points, because that's enough to block out some attacks but not enough to make an untouchable defensive powerhouse. The ones with a higher defensive ceiling have drawbacks beyond their Energy cost. This makes sure even defensive powerhouses have a weakness that can be exploited and the action never quite grinds to a halt.
The inspiration for this Upgrade were the various defense systems in the Gundam franchise like Phase Shift Armor and I-Fields. To help represent those, Absorbing Armor started out only allowing you to choose between Beam and non-Beam. Playtesting showed very quickly that trying to stop Beams this way wasn't likely to block the whole attack, making the second half of the ability pointless, so the choice of specialization was freed up to any of Melee, Shooting, Beam and non-Beam. The problem with Absorbing Armor is that there's way too many offensive buffs in the game for it to reliably block attacks with such a small Defense boost. It works for very specific builds like Guardian of Steel users and Combiners, which can trigger the 3 Defense at no Energy cost much more often. It also works okay with Experimental Reactor builds that have a secondary focus in defense. The full block won't happen very often but they couldn't afford to spend 2 Energy per attack anyway. Perhaps it should have given a larger bonus but with a drawback of some sort? Overall it's not great, but it's okay.
This was the first Active Defense to be written, and it is the most effective per point of Energy spent. Five points of Defense is enough to block out buffed low rolls and unbuffed high rolls. Originally you had to choose between Melee and Shooting for Specializations, but it got changed along with Absorbing Armor when that one wasn't working with Beam and non-Beam. The worst you can say about it is that it is kind of boring, but it more than makes up for that in effectiveness and being easy to reflavor to whatever you wish.
This started off being equal to Electronic Cloaking System, using all your Speed instead of just half, but Speed is generally more useful than Systems so it needed a drawback. The nerf came in the form of a halved bonus so that Boosted Lance users wouldn't be too difficult to handle in large maps. This Upgrade's problem is that it'll just grant 2 Defense for the first few Power Levels. It only costs 5 though, so it remains effective.
The most unique Active Defense in the core rules. This started out as an all-purpose barrier that let you respond to attacks with a Defense bonus like all the others, but that proved very difficult to balance. With a 1 Energy to 2 Defense ratio it was very easy to make yourself invincible against anything that couldn't punch through barriers, but 1 Energy for 1 Defense was just plain terrible. Then I wondered "What if I make it an upfront investment as a balance mechanism?" and that naturally led to switching a Defense boost for Damage prevention to make it easier to track and play with. Absolute Barrier is a strong candidate for Best Active Defense, in competition with Custom Defense, simply because the adjustable output and ability to block direct Damage are just too useful. I would have liked to make more preventive barriers in the core rules, but they're tough to get right. When I tried to make a bunch more for the expansion, it ended in only one more (Organic Barrier) making it all the way to print!
Electronic Cloaking System
This one grants a very large Defense bonus so it has a weak point in Blasts, Bursts and Lines. Without that, Support Mecha would be too frustrating to fight against. I do like that, conceptually, the robots that fight indirectly or stay away from the front lines are the ones who can use invisibility defensively. With that said, this Upgrade suffers at low Power Levels, because you need a ton of points in Systems to keep it from being "Custom Defense, but worse". While ECS is kind of weak in Core, the Expansion added Dispersion Aura to defend specifically against Blasts, Bursts and Lines which covers this Upgrade's sole weakness. It is an expensive combination, but probably worth it if you're worried about survivability.
The Defense bonus is small, and in general it is more efficient to just let everyone else use their own Active Defenses than to buy an expensive one and give other people just 3 Defense with your own Energy. But with some planning, this can allow Squadmates to focus on stronger attacks instead of spending their resources defensively. It is especially useful at low Power Levels when MP is tight and most builds could use it on something other than Active Defenses.
One thing that jumped at me while doing this was that there's there's a lot less Active Defenses than there are Weapons. If they're supposed to be their defensive equivalent, then the game ought to have a few more options, right? Not too many, but at least two pages total rather than just one. Perhaps keep one page for the Defense boosts but give a whole page to ones that prevent Damage. Fortunately, the expansion does a very good job of, well, expanding Active Defense options but I figure there's still room for a handful more in the game.