People are being very disingenuous when they say Zhangye Danxia is not as good once you have a few encampments up. That's not how opportunity costs work, Zhangye Danxia doesn't stop doing what it does just because you can now get Great General points in other ways. Zhangye Danxia gives you +2 Great General Points, which is equal to an Encampment with a Barracks. That means that if you have Zhangye Danxia, you can have one less Encampment with Barracks compared to if you didn't have Zhangye Danxia, and still have the same Great General output. If you have one less Encampment with Barracks, you can have one more Campus with Library, or one more Holy Site with Shrine, or one more Theatre Square with Amphitheatre. In other words, you can think of Zhangye Danxia 'converting' from +2 Great General Points to +5 Science for a reasonably well-placed Campus with Library; because if you didn't have Zhangye Danxia, you'd have had to have built another Encampment and Barracks instead of that Campus and Library. That's really powerful, especially because Districts and District Buildings give those yields without actually being worked. You can only get +5 Science from Yosemite by working 3-4 otherwise mediocre tiles, which is a huge commitment.
Secondly, Giant's Causeway is also limited to very early military stuff. Saying 'Zhangye Danxia gives a boost early, Giant's Causeway gives one consistently' is just super misleading about how the military game in Civ VI works. If I took the time to march my units all the way from Giant's Causeway to the frontline for the whole game, I would be slowing my Domination Victories immensely - once I've finished warfare in the immediate vicinity of the Giant's Causeway, it can take 10-15 turns to march troops all the way over. I value a unit immediately on the frontline when I need it much more than a +5 combat bonus. Giant's Causeway is good if you find it early and it can help your initial starting war, but any later units are probably going to be purchased by Gold on frontline cities and have nothing to do with the Giant's Causeway. I think if you are still wasting time to do a big "stand next to Giant's Causeway" circuit, you are playing very inefficiently and are not actually seeing half as much benefit as you think you are.
Finally, the early Great Merchants are being underplayed. Zhang Qian and Marco Polo provide an extra trade route, which can be very powerful early on, Irene of Athens an extra government promotion, and from my experience you will get all the early Great Merchants if you lock down Zhangye Danxia. "+2 trade routes and +1 government promotion" is a huge "yes pls much powerful".
For the downvote, I want to do Yosemite again because it is still being disgustly overrated for something that provides a thin smattering of Science and Gold, but to avoid a double downvote, I'd like to move to the Matterhorn. I agree with some posters above re: thin yields starting to be a problem for Natural Wonders like Tsingy de Bemaraha, and if it wasn't for Yosemite, Tsingy would be nearing the bottom of my list. However, there's one in-between Tsingy and Yosemite for me, and that's the Matterhorn. Why? 2 reasons. Firstly, Tsingy tends to be free-standing and all the adjacent tiles are workable. Matterhorn spawns in Mountain ranges predominantly, which means that it often ends up providing benefit to only 3 or so adjacent tiles, which is a lot less. Second, it adds less to its tiles than Tsingy, and supposedly makes up for it with the Hills movement bonus. However, this has the same problem as Giant's Causeway - I'm just not moving all my troops over there in the mid-game - but isn't as good as Giant's Causeway because your foe won't necessarily have hills predominating early on. My mental picture of the bottom Wonders now includes Giant's Causeway, Matterhorn, Roraima, Yosemite, and Tsingy de Bemaraha, so I want to draw some attention to Matterhorn and put it in the fight with GC and TdB.
Eyjafjallajökull [17]
Giant's Causeway [5]
Ik-Kil [22]
Matterhorn [5]
Mount Roraima [23]
Païtiti [30]
Pamukkale [11]
Piopiotahi [18]
Torres del Paine [23]
Tsingy de Bemaraha [4]
Yosemite [13]
Zhangye Danxia [6]