Mass Effect is a good example of the weaknesses of the trilogy format when impropery used. For example, Mass Effect 1 is after all just Mass Effect, the game was designed in such a manner that it could function as a singular story if there wasn't enough interest or didn't perform well. The Reapers would just be forever trapped in darkspace. the narrative is purposely ambigious about future events with minimal foreshadowing on what would happen in future installements . What little foreshadowing there was present in ME1 was largely abadonend in ME2, instead the developers chose to reinvent the universe through a series contrived plot points. The game itself shifted focus from underlying themes such as the rise of humanity to as series of character-centric narratives, with minimal progression to the overarching plot. Come Mass Effect 3, where player's are suddenly thrown into dark, but poorly thought out war story , the premise and opening the third game had almost no connenction to the previous games and might as well function as an independant story in it's own righ.
Put bluntly, the Mass Effect trilogy was an experimental mess, with each game having radical differences in approach in tone and style compared to the other games, in retrospect, on a design level the series lacked any form of coherence.