The issue is that Grete A: Is competing with Beat Cop (2) which is probably on the top 5 list for 'best ally in the game' and B: Has some logistical issues with her clue gathering.

Once you get past a certain point as a team/player, the challenge you are optimizing for stops being about 'getting enough clues to advance the agenda' and becomes 'we need to clear victory locations.' If you happen to be on a victory location for the kill that is nice, but Grete won't change the rate of clue gathering for good clue gatherers you with that much, and it isn't a consistent way to clear a location on your own.

Usually the movement taxes you face are a bigger barrier than your clue gathering rate when it comes to clearing high clue locations, in 2 player it is actually common enough for you to spend as many actions entering and exiting a location as gathering clues there! So Grete sorta fails as a guardian flex tool in that she doesn't easily let you run off and gather clues yourself, she gets you a nice little bonus clue for protecting the seeker, but it isn't as good an acceleration tool for the team as say... your combat Finn popping Pilfer and using their lockpicks to completely clear a 4 clue location by themselves.