Evangelion Units are pretty impressive in what they are capable of, and can be a game changer in the world of Angels trying to end the world one attack after the other, but they are greatly flawed. One of the biggest flaws, in my opinion, is the gratuitous harm inflicted to the pilot should the Evangelion suffer damage during combat, to the point where pilots actually, physically lose parts of themselves, in reference to Asuka's eye literally bursting open when EVA Unit 02's head is impaled with a spear of Longinus.
Now I know that Evangelion Units are instrumental, no pun intended, to SEELE's master plan of starting Third Impact, seeing that the EVA Units are clones of various angels, primarily Adam and Lilith. That is all logical to me at least, a catalyst is needed for a plan this big. But the EVA Units have some major drawbacks in their very design and concepts, for instance :-
- They are cybernetic in nature, seeing that they are organic beings with souls made of flesh and blood "enhanced" with various technological upgrades, such as weapons and armour plating. Some implants and "enhancements" may not work as planned or not at all, and could have an adverse reaction to both the Unit and it's pilot.
- The EVA Units are somewhat unpredictable, and have been shown time and time again that it can go beserk with the pilot still onboard, often acting without any consent from the pilot and/or external forces.
- Flesh does not generally go well with combat, meaning that any damage sustained to any limb during combat has the potential to put the Unit out of action until the Unit either heals itself, or repairs are done afterwards, due to the pain caused to the EVA and to the pilot themselves.
- EVA Units need a soul in order to operate correctly, as seen in the test phase of Unit 00 in the Anime, before combat preparations can even be considered, meaning that soul harvesting and other preparations must done prior, leaving the issue of finding a viable soul for said Unit.
- Evangelion Units need time to grow and mature before being prepped for combat, as seen with Evangelion Unit 05, which can prove to be detrimental to an ever growing war effort
- The cost to maintain EVA Units are not justifiable when compared to normal mechs, as EVA Units need special environments and equipment to be cared for, such as that red liquid that all EVA Units are submerged in when they are not in combat, to the many restraints in place around and on the EVA
The list is somewhat long, but these are the main issues that I found with the Evangelion Units.
Compared to the mechs from, let's say Pacific Rim for instance. The EVA Units fail in nearly every place where a mech should be focused on, like effectiveness in combat. The mechs in Pacific Rim can be readied for combat at a moment's notice, fully combat stocked and all, with little to no harm present to the pilot whatsoever from the mech, apart from major damage to the mech, and are almost guaranteed to work the first time, as opposed to the EVA Units. The Jaegers essentially fill all of the EVA Units shortfall, because they are fully mechanical in nature. Not only that, they can be built in relatively short periods of time, being fully combat ready at the end of production, as well as being more predictable in combat when in combat.
One of the answers that I would disregard is the EVA Units ability to generate AT Fields where normal mechs cannot, considering that AT Fields can broken with large amounts of energy OR other AT fields, plus adding the fact that the creator of Jet Alone said that artificial AT fields could be generated through fully mechanical means.
From a purely tactical point of view, why even consider using Evangelion Units over normal conventional mechs in the first place? What benefits do Evangelion Units possess that normal mechs do not have?
I mean, the Angels only started attacking Humanity again 25 years after Second Impact, surely mech design would have progressed far enough to build fully mechanical mechs to counter these threats, as well as developing methods to generate AT Fields artificially.