I saw or reddit that we should write feedback here to get noticed, so here goes.  Some of these will be repetition, I figure saying something more doesn't hurt, but possibly missing something does.

Bugs/Buglike issues

When cultures transcend, they are reported as an era early.  (as an example if, say, Aztecs stay Aztec into early modern it says "Aztecs have kept their traditions in the medieval era" or such.)

The main map city merge button doesn't do anything unless an unmerged outpost is in contact with (one of?) the merging cities.  Another merge button does seem to work, though see point below.

If I move and attack with my other units, and have possible reinforcements available, the battle moves to the other side without letting me reinforce.  Having the next turn pause to let me reinforce if possible would be useful.

The Landstaker(?) achievement, where you finish the game while controlling more that some amount of territories, doesn't work.  I've won a few games where I control everything, or everything outside of 2ish territories, and still didn't get it.  (Landstalker in particular because i would like my AI persona to grab as much territory as possible.)

Smaller Recommendations/Problems

Island territories should have a way to build on the separate islands.  You can do this with garrisons, if the islands are all multihex, but it would be simpler to be able to just build normal districts on them somehow.

A better way to combine cities or rearrange cities after conquest would be useful, right now the "build units if you want to save the population, ransack, build outpost" is kind of clunky, and it feels weird to basically genocide a city for a gameplay mechanic.  Maybe allowing cities to downgrade to outposts while keeping the population, and/or having a "migrant" unit that can easily move population to other territories, or something along these lines would help when conquering and merging/playing with territories, especially after a conquest.

Speaking of which, outposts seem to grow and shrink.  To avoid clutter, seems they should grow and stop growing instead.

Alpini in some cases are better than rifles (slightly less combat strength, but the ability to climb hill is very useful), so they should be able to stick around.  Same may be true other other special units.

Sometimes, I can't do a unit upgrade where the obsolete unit jumps 2 steps ahead when I don't have resources.  In these cases, being able to upgrade 1 step to the obsolete unit 1 level back that doesn't not require the resources.  (An example is swordsmen--> great swordsmen --> musketeers,  If I don't have gunpowder, I would still like to upgrade to great swordsmen. 

Larger Recommendations/Problems

AI isn't that good at fighting battles.  Unless I'm against very tough units (Hello Huns.  Or elephants in another game.), I rarely lose much of anything if I organize the battle at least somewhat well.

The game weirdly feels too long and too short at the same time.  Too short, as others have said, because eras seem to pass too quickly, especially ancient, to really breathe and enjoy them.  Too long because, like a lot of 4x games, it just ends up easily sucking up hours of time, even to do relatively basic stuff in game.  Obviously, both of these are tricky to deal with at the same time, so for some smaller parts of this complaint:

Faster unit movement would be nice, or at least an option.  I can pretty easily queue multiple movements and attacks at the time time of the movements are relatively straightforward, speeding up how they play out would be nice to spend less time watching things happen.

Slowing down techs and era stars compared to production, maybe speeding up unit production a lot, would be helpful.  It almost feels like a 350 turn game at what is otherwise normal speed would be the sweet spot.

I've mostly stuck to what I'll call the Master of Orion 2 strategy of civ picks, going some combo of science/builder/food with maybe an asthete thrown in for happiness.  I'll back up others that this just seems like a generally obvious way to pick, apart from a hun or mycenean threat early on, this type of civ just runs over everything.  And Harappans did in fact seem OP when I finally got them (Vs. Egypt and Zhou.)

I see the different naval terrain, but it didn't see to effect naval battles much, which didn't come up much at all.  Navies I'm guessing will get more work.

Pollution, as others have said, could also use work.  I've just not built anything that causes pollution to avoid any issues.  It basically seems like a way to end the game quickly in a blaze of glorious extra production.

Unique districts buildable 2-3 eras after the culture seems useful, sometimes shorter eras make it hard to build these.  (The worry I heard was over, say, Egyptians building Pyramids in modern times, but 1-2 eras ahead makes sense especially with most, say, industrial buildings.)

Map size large seems too big (Colonizing the new world seems a lot to swallow and leaves one metaphorically painfully full), map size medium seems too small.  (I've been able to pretty easily conquer the world or come close both times on this size)  Custom map sizes would be nice, unless I missed the option.

The weird unit upgrades age to age are a bit tricky.  (I'm talking how spearmen and archers have no classical uppgrade, the swordsmen line skips to flintlock, knights stay early modern, etc.)  Seems slightly weird balance wise, and that either more unit tiers should be added, or units in different ages made the same tier of power.

Comments Neither Good or Bad

In most of my games, whoever is pink seems the most powerful for whatever reason.  I think the AI for most of them was Elissa, and an extra game was a coincidence.  Not sure if this AI is in fact powerful for whatever reason (is rated as normal), Or if it is a funny coincidence and the color pink likes to provide a challenge.  (All times with the same AI it picked asthete cultures + Aztecs, so I am pretty sure this is a strategy it uses.  the different AI was just a coincidence, was a different persona and different culture picks.  Still including Aztecs though, maybe for AI aztecs are OP)  In all of these games, said pink AI was the only one competing evenly with me, other cultures were left behind.

I like those 15 science osmosis events in modern times.

Good Stuff

The combat system is fun even with AI limitations.  Finding ways to beat those tough units with few casualties are a good challenge.

The district system and building cities is quite fun.

What I've seem of modern vs. modern combat was also fun.  There was a lot of snowballing so I was ahead, but defending a city with partisans, dealing with a little artillery, and such was enjoybale in what i saw of it.

Additional thing:  Lots of stability late game makes stability pointless in modern times.  Seems like something should be added to make things useful.

Pink was the most powerful in my games too. There is an easy explanation: A bug. Specifically, the one were pink, the third player, gets patronage over time for every independent that spawns a city anywhere on the map. 

Latest bug report: https://www.games2gether.com/amplitude-studios/humankind/forums/215-bug-reports/threads/43782-pc-steam-beta-patch-gamebreaking-patronage-bug-still-present

That would explain it.  Thank you.  :)