I recently purchased a new computer, an iMac Pro, and have noticed several major graphical errors in FFXIV that I did not have with my previous computer, a much older Mac Pro.
The most consistent error comes when rendering the textures of some character model skin (usually bare skin in certain cut scenes, but occasionally visible in the world at large), moving water in some lighting, and every instance of mist or fog beneath ground level (for instance, over the edge of the islands in the Churning Mists). In this error, the textures seem to be replaced by a grid with the actual texture shrunk to a tiny point in the middle of each box. Geometric lines on the model (which I presume to be the edges of rendering polygons) are unaffected, along with a little area around each. It's hard to describe, but I've been taking screenshots if that will help. The character texture effect is consistent for a given cut scene regardless of my computer's state at the time (i.e. freshly rebooted or on for a while, many programs open or no other programs open, etc.)
Also, I will sometimes get other graphical errors upon entering a new zone. Sometimes, this results in the HUD flickering a few times, and then vanishing entirely. Other times, it will cause lighting effects to be displayed as huge flat-color polygons radiating out from the light source. If it occurs when zoning in to a dungeon where I haven't yet watched the intro cinematic, it will briefly show the players at the start of the dungeon, then flicker in to the cinematic, and when it returns to the world both the lighting effect and the missing HUD will be in effect. In all these cases, if I tab out of the game at that point, it crashes.
I've tried adjusting my graphical settings several times, and have not yet found a combination that addresses any of these issues, though I probably haven't tried everything yet.
My specs:
iMac Pro (2017)
macOS Mojave (10.14.5)
Processor: 3 GHz Intel Xeon W
Memory: 64 GB 2666MHZ DDR4
Graphics: Radeon Pro Vega 64 16 GB
Display: Built-in Retina Display, 27-inch (5120 x 2880)