TLDR: The Library on Legendary is too easy, either due to AI or sandbox changes. Please make it harder.

Finished coop Library on legendary; both me and my coop buddy (who's a casual) felt it was too easy compared to vanilla. We replayed vanilla to make sure and yeah, vanilla was much harder.

Below is why I think this might be.

Vanilla doesn't leave any room for messing about. You have to (i) keep moving lest you get swarmed, but (ii) be cautious not to blindly step into rooms full of hordes, instead clearing them first, and (iii) take risky detours if you spot shotgun ammo to pick it up.

The mod gets rid of (i) and (iii). The flood don't swarm you as easily because they don't keep respawning as much and they feel weaker in general (they don't casually dump an AR mag with high accuracy to kill me). Shotgun is also (thankfully) not the mandatory weapon of the level anymore, so you don't have to walk into the enemy lines to pick up shells. In this way the flexible sandbox of the mod puts the player in fewer dangerous situations.

The mod let my friend casually explore all corners of the map, whereas in vanilla taking a wrong step could be a death sentence so we had to strategise hard. I think vanilla is how legendary should be, at least for this specific mission.

It's not a fundemental problem with the mod either; the Two Betrayals secret was hard af. The modded campaign can deliver a difficult challenge if it means to.