I personally appreciated Firaxis experimenting alot with all of the Frontier Pass stuff.
I think the New Frontier Pass DLC was in general well received. As you mentioned, a lot of the new civs/leaders were unique and well done (although I believe they already started reusing art assets from Amanitore/Nubia for Lady Six Sky/Mayans)
But the game modes, while fun, were superficial with awful QA
- the Monopolies and Corporations mode had a game breaking bug where the AI wouldn't improve luxury tiles that was left unpatched for years (while modders had a hotfix within days. But fuck consoles, right?)
- they made a Heroes mode with super-powered legendary Heroes units where the AI couldn't actually use the Hero abilities against the player, causing insane imbalances
- I can't think of any Apocalypse Mode bugs, but it was generally a mode that doesn't get played unless someone wants to meme on meteors destroying entire cities late game
- the Raging Barbarians mode is a great addition. But still had QA stuff where converted City States from Barb Clans won't recognize or fight existing Barbs and vice versa.
- Tech & Civic Shuffle Mode was well done and improves replayability
- Secret Societies mode was popular but also unbalanced. The AI can't handle the mode, building and filling up the screen with useless Cultists for one.
- Zombies mode in general was hated for how superficial and unfun it was as well. Zombies ignore Settlers and Builders, so they can be gamed by building human civilian shield walls. The new traps and fence tile improvements were generally useless because you 1) lose tile yields and 2) Zombies still spawn randomly behind your defensive line, making your barriers useless. The Turn Undead City Projects also create endless positive feedback loops where converting zombies counts as zombie kills, so recruiting zombies is pointless because it just creates stronger and infinite spawning enemy ones.