Neither of us is experts at using espionage
Very true!
But, the pedia entry for spy says "Helps Thwart Rival Spies".
I interpreted this as just meaning that they have a counterespionage mission.
If you see BG has a significant lead in EP, then you had better run some counterespionage.
The problem with counterespionage (the mission) is that if the enemy *isn't* running any missions against you for the next 10 turns, you are actually now in a weaker position for any future missions than you were before, because you have spent EPs against that player on the useless counterespionage mission, so enemy spy actions against you are now cheaper.
Counterespionage is very poorly designed.
I recall this was a comment about the Loki unit in FFH at one point. He also has a city flip ability.
Keep in mind that Loki can flip only cities with 0 culture, not mutliple thousands of culture powerhouses.
It does seem that BG city flip should have some diplomatic fallout. Perhaps a -4 diplo penalty, "You converted one of our cities!"
I'd support this, if we kept city-flips.
But, it seems there are three city locations that would have different effects:
Despite Sylvnn's reports, I'm not seeing cultural city-flips, cities are just too spaced out.
Also, converted cities are converted with their economy and infrastructure completely intact, so its pretty easy to have them build culture (or temporarily flip your culture slider) and have the city expand a couple of levels in only a few turns, which stops any flips.
Is there some other way we can use this effect, which is flavorful and not too painful?
I think plunging a city into civil disorder for many turns, or plunging a civ into anarchy/uprising are probably more flavorful and easier to balance. I can't really see the Bene Gesserit managing to convince a city to abandon its House.
I guess the actual way of interpreting this would be as a political move; they manage to convince the Emperor/Landsraad to reassign the fief ownership of a particular city to that of the BG's proxy house (ie to the BG player).
Kinda like the 19th century Great Powers assigning colonial rights to various territories between each other. That kind of thing is definitely in the book (how the Atreides are assigned Arrakis instead of Caladaan).
Maybe it would be ok if it:
a) It were very expensive in EPs
b) We could give some kind of flavor indication of what was happening. Maybe rather than "convert city" it would be "reassign fief" or something.
c) Capitals were immune
d) There was some way to prevent it. Eg: the mission cost scales more with relative EP investments, so you could maintain a block by having enough EPs against the BG player. Or it couldn't happen to any city with a spy unit in it. Or the cost of doing it was heavily increasing in the culture level of the city.