Personally, I think the biggest issue with the game is the Exploration age. Its mechanics suck and are mostly contrivances. It is sadly also the age that lasts the longest.
So in that sense, UrsaRyan is absolutely correct: Specialists are flavourless, Missionary Spam is boring and braindead, Treasure Fleets are 100% luck dependent. All three tie into Exploration.
Exploration Religion is a stain on this game, and should be treated the same way Ideology is treated in Modern - as a diplomatic (not cultural) mechanic that helps bring players together or further apart, so that Exploration diplomacy isn't a pure extention of Antiquity relations. Ideology itself could be improved (by for instance making the Ideologies themselves have friendships or rivalries, rather than a strict system where everyone hates anyone on a different side - Democracy and Communism were able to work together to tackle Fascism, remember.)
Replace the Relic system with Fine Art, which is made by Great Artists that you attract by entering Celebrations. Or something like that.
Suzerain bonuses that give you Free techs upon becoming the Suze (another thing Ursa correctly points out as 'not balanced') are so potent you rarely initate endeavours early into the Age, even if you get a bonus towards Endeavours. In my current game, I've rolled a Layafette + Greece start, and used my unique Endeavour twice. Twice. Across all three ages. Suzing City states, improving Trade Relations and acceping Research Collaborations from the scientific leaders have all been way more important than just +1 social policy slot (which the AI also gets if they accept, so never ever offer this one to Himiko).
Aside from Exploration I'd say the worst mechanic in the game are the Crises which add nothing positive to the game. No stakes, just annoyance. Praise the heavens you can switch those off.
I WILL say though, Antiquity and Modern are both good, and very fun, having played through the game a few times now. The victory conditions aren't fully balanced - Cultural and Economic are much easier than Militaristic, which is significantly easier than Scientific. It needs a bit of balance tweaking, but that's fine. The AI is absolutely able to win Cultural or Economic if you aren't careful.
The Civs themselves are also excellently designed, and each of them plays very differently. In my current game I went Greece => Ming => Mughal and each of these was completely different, which made the game interesting. Going from the brainy Ming into the greedy but stupid Mughals was a surprisingly pleasant whiplash, which forced me to adapt myself to Mughal's economic strengths, and use the gold to inflate the Culture bonuses I inherited from Great Walls, Acropoli and Layafette's Attribute tree. No, it isn't a straight line. But you do have to *think* about your plan, and the game rewards you for making good strategic decisions. Which is how it should be.
The game itself feels easier but also more fun, and large part of that is the low cost that units have. Pumping out units and maintaining them is way easier in Civ7 than it's been in any other Civ game, and that keeps it dynamic. You can always invade enemies, and defend yourself by buying units in a pinch. So can the enemy. War has stakes, and the classic steamroll like you'd have in Civ5 and Civ6 simply doesn't happen in Civ7. Usually, at least one AI will be able to snowball into a huge tech lead up to the max of their Settlement limit (by my observations an OP leader like Tubman or Himiko, or whoever is lucky enough to start as the Maya) and become a threat to win.
I dunno. The game has changes, and sometimes those changes feel like net neutrals. The game itself though? Absolutely playable, and good. Underrated if anything.
It's the Heroes 4 of Civ.
