With six players and their combined talents, in ship combat if I get into a spending war with threat verse momentum + ship power I will eventually lose. They can earn it and multiply it’s effects better than any NPC ship though in “Boss” fights I make them earn the win. Second, I would not dare introduce a fluctuating shields like complication, cause anything I can do with threat my players will expect to be able to fix or inflict with momentum. Two threat is 4 points of penetration. Using two threat to generate a 30% chance to negate 20+ resistance per attack sounds in my opinion arbitrary, unfair and petty.
A Borg would flip our entire campaign on its end and not in a good way. The Borg in my opinion are like the Omega Particle, when they show up everything else becomes secondary. I think it would be distracting for the intricate interstellar political wild west gold rush style frontier game that I am running and would feel very heavy handed.
I am fine if the rule is that Shield Modulation stacks. I am not out to kill my players or blow up their ship. If they put in the extra effort to keep pushing up their resistance to the point where one of my villains has to admit that they can no longer damage the players through their shields and back off, then more power to my players and what a Starfleet way to drive off an enemy. It just means my villains will have wait to return in another scene to try again. Maybe when my players have had some bad dice luck and haven’t had a chance to get a full momentum bank or when the villain is attacking a target they need to defend so the players have to be more aggressive. Maybe my villain needs to pick up some polaron weapons. I am fine with that. It makes for better villains and better stories, it also means my villains will have to shake up their attack patterns, engage in long term plots of black mail, or sabotage, or lure the players into a nebula where shields are useless. I am not going to punish my players for being creative, playing the rules to the max and designing and investing in their ship. A villain could return the favor which could be a serious challenge for players to over come if say a transporter lock is needed for some mission critical goal.
In the end it is not me verses my players, I am the narrator of a story and a referee of a game. I want tension to build naturally from the events unfolding in consistent manner with hints, clues, and cues of impending danger built upon the rules of the game. If they “lose” against a villain it is because they mismanaged their resources, made a mistake, the dice were out to get them, or they took a risk and it didn’t pan out. That type of failure feels natural, that they were bested fairly and makes them look forward to the next challenge, instead of plotting my death because I suddenly awarded a bunch of arbitrary points to Gryffindor so they win the house cup over Slytherin. (love the memes)
The fact that they had an option to “turtle” was one of the things they liked about the system. The expanded complication range is a risk they take every time they divert “maximum power to shields” usually followed with evasive maneuvers. It is not perfect, the activation roll has bitten them in the butt once with a complication and another time they failed the check outright. But it gives them more options in how they deal with situations and gives them a heads up on their enemy’s intentions based on how they respond. Does the enemy close, scan for weakness, modulate their own shields, go for a power shot dropping all the threat in for penetration, do they cloak, to they go into an attack pattern, do they go evasive. It allows them to take a more defensive and diplomatic stance without being punished for not firing first every time and I am fine with that. This is a Star Trek game after all and they are Star Fleet. 
