well, I can't speak on the Emi Theater design, but the direction I wanted to go with NERGAL was definitely paying attention to a few factors. Primarily, parsimony of mechanics/ease of play, paying close attention to action economy impact, and availability of both synergies and counterplay
Something that I try to strive for with design is stuff that's very straightforward in terms of how it works, and hooking into existing design, in order to keep it conceptually focused and easy to use. Using Grunts was a very obvious choice for a fragile minion, and from there it was considering how to take a very simple mechanic (a wreck becomes a grunt) and balance it without making the mechanic itself overly complex.
That's how I ended up with the Full Action requirement (to balance the potential Full Action you're also getting out of the grunt). On average it takes somewhere between one and two quick actions for an NPC to get rid of a grunt, so in a white room it's zero-to-negative action economy gain, but in practice you can boost that fairly easily. The most effective techniques are timing it within the activation order so you can guarantee being able to use it on the same round, only targeting wrecks that are positioned in a way that's tricky for an NPC to deal with (cover, LOS, range, defender allies), and choosing which wreck to raise carefully in terms of NPC type (survivability traits/systems are especially good).
Having a decent number of charges which you can easily boost with ENG investment was also important to me because, being grunts, raising them can be quite swingy - all it takes is one lucky shot or damage that can't be mitigated to put them down, so you're never guaranteed to make back your action investment. Trey solved that issue by giving them native overshield instead, which is also a good solution.