Quite new to EL, and I was just looking at some of the DLC, and "Monstrous Tales" seems most interesting because I like quests -- but the Steam Store has slews of reviews suggesting that the DLC is broken, more or less. It has been some time and now the studio has moved on to Humankind, but has MT been fixed in the meantime or is it still considered problematic?

The item is on sale on steam at the moment, I was wondering if there is any consensus on this. I am assuming the game is no longer in development (as is ES2), since the team has long moved on, and no more patches will be forthcoming.

MT at launch was a mess and I don't know how much Amplitude has gotten to it officially (they folded some of ELCP's bug fixes into the base game with MT, but I don't know that they took any of the subsequent fixes that actually make MT playable and fun).  

Highly suggest you check out ELCP here - https://www.games2gether.com/amplitude-studios/endless-legend/forums/15-modding/threads/30618-endless-legend-community-patch?page=1

The two online MP discord only use ELCP, the game is not great without it for various reasons.  But turn down difficulty 1-2 notches from what you currently enjoy, the AI becomes better at playing the game and competing.

TL:DR - Yes buy Monstrous Tales, but I can only verify it is good when running ELCP.

Ok thanks I'll check that out when I'm done with my first game. Finding it enjoyable despite not having this elcp. If I understand correctly it also involves downloading an installer and altering core game files (so no mere workshop mod).

Correct, you can install via the .exe installer to be lazy (we all use it for that reason, lol) or there is a .zip version where you can examine the contents of the mod first.  I believe it wasn't possible to mod the game as needed via the steam workshop support, it edits core game values in files.

If you want to uninstall it, the .exe version is available under windows for removal or you can Verify your steam cache at any time and 10mb of files will download and make you Vanilla again.