Essay: the physics and exploitation of mass effect fields
As the name implies, this is an attempt to analyze the possibilities opened up by eezo both considered by the game and not. I will start with FTL (and the absolutely terrifying weaponry derived from it), then the technology seen in the games, before going through various technologies that the games failed to explore.
FTL: destroyer of worlds
The mass effect FTL drive works via reducing the mass of everything in range, which both locally increases c and makes the ship capable of getting kore delta v out of a given unit of fuel. Assuming that the same mass fraction can reach the same fraction of c after mass shifting is taken into account, that civilian ships travel at 3000 (external) c, that the unaltered delta v of a civilian ship is roughly 3,000 km/s, and that to save fuel only 600 km/s worth of fuel is burned per normal interstellar transit reveals that the internal speed of light within the mass effect field is approximately ~900,000,000,000,000 m/s compared to the normal ~300,000,000 m/s in the rest of the universe with the ship traveling at ~900,000,000,000 m/s . But how does this destroy a planet? The answer comes from mass accelerators, in them a projectile's mass is reduced to allow it to be propelled to an otherwise impossible velocity (by all rights the mass effect universe should be a lot more powerful than it is) the key however is that when the slug exits the barrel (and its associated mass effect field) it retains its previous velocity, but its mass increases gaining kinetic energy from effectively nowhere.
In our hypothetical 3,000 c example, if the ship normally masses 200 tons and hits a planet at full speed, it will deliver energy equivalent to a bit more than 2 megatons of TNT (assuming the mass effect field remains intact until the ship's kinetic energy is more or less spent). The scary thing happens if the ship's mass effect field blinks out immediately before or during the impact, for reasons I will detail below. (and assuming relativity remains functional)
When the mass effect field blinks off, the 3,000 c ship should stay at its present velocity from all perspectives, but moving FTL is impossible under relativity. Therefore the ship suddenly switches to high-relativistic velocity with a time dilation (Lorentz) factor of 3,000 (roughly). To put this into perspective, if the Lorentz factor were 2 (which happens at 0.86 c for those interested) the ship would hit as if its entire mass were converted to energy on impact, which comes out to 4 teratons for a 200 ton ship, for those interested. I shy away from trying to calculate the kinetic energy of an object with a Lorentz factor of 3,000, but remember how Taetrus was hit by a ship at 'near FTL' speeds in the back story? By all rights Taetrus should no longer exist except possibly as an asteroid field.
Getting away from the prospect of one-shotting reapers by the 1,000 with a relatively cheap weapons system the size of a tissue box, I will discuss other issues with mass effect FTL, starting with charge. Charge is just silly, as it is already possible in real life to convert static electricity to a useful current. The biggest problem would be provisions. You simply must realize that unless a ship is large enough to sustain agriculture (like a Quarian liveship) or you are robots (like the Geth), the distance you can travel is limited by 2 things, namely 1: how fast your ship can go and 2: how long it will Be until the crew eat all of the available food. With the Relays (those foul Reapers) removing much of the reason to explore and the general scarcity of garden worlds, it makes sense that an insufficiently paranoid civilization would develop into the sort of low density empire the Reapers need to be victorious.
However an extremely paranoid civilization would likely put any relays they found on a collision course with the nearest black hole and become an extremely high density empire grabbing everything in range and living in hollowed out asteroids or other space habitats if a terraformable or gqrden world were unavailable.