Too many civs that need buffs in my opinion, so I'm not gonna bother listing them all.
I'll just start with a random one:
Inca.
They are an ok civ, but fundamentally I feel like they tried to cram their civ 5 strengths into civ 6, where they don't fit.
Granted, Incas can get some ridiculous food combined with production from the terrace farms and mountains, but this isn't civ 5 - you can't make good use of all that food and production since you don't inherently need a high population to begin with in this game.
In civ 5? Absolutely, your science for instance scaled off the population base numbers, as did your happiness if you went for Tradition - but this isn't civ 5.
It gets worse too, because those mountains are good locations for plopping down campuses and holy sites, but this is at odds with their only real strength - high mountain adjacencies to their terrace farms.
Their UU is pretty trash as well.
Its ok on its own stat wise, but why would you ever want to heavily invest into the scout class to begin with, even if you do get a temporary (and weak) power spike? They turn back into trash again if/when you upgrade them, so it's not really worth investing into their UU.
Otherwise, the Incas have no bonuses toward any win condition, and that's pretty crap.
Heck, people talk about Georgia and Spain being crap, but at least they have some (if underwhelming) bonuses toward a win condition over a vanilla civ.
Incas just don't, and its sad to see their civilization being this bland and without any clear direction.