I agree that the support for uncommon resolutions is less than ideal. That said, I've gotten it to work. Caveat: sample size of one.
On a 2019 MacBook Pro 16" with the dedicated GPU, I've had success connecting a 5120x1440 Samsung CRG9 via DisplayPort. I had to use this adapter [0] and this cable [1] to do so, but it does work. It also works on a 2018 MacBook Pro 13", but due to the lack of a dedicated GPU (i.e., Intel graphics only), the output is a bit fuzzy as it's not a natively supported resolution. Still, it's good enough for productivity work, though not for video/photo editing or gaming.
On a 2019 MacBook Pro 16" with the dedicated GPU, I've had success connecting a 5120x1440 Samsung CRG9 via DisplayPort. I had to use this adapter [0] and this cable [1] to do so, but it does work. It also works on a 2018 MacBook Pro 13", but due to the lack of a dedicated GPU (i.e., Intel graphics only), the output is a bit fuzzy as it's not a natively supported resolution. Still, it's good enough for productivity work, though not for video/photo editing or gaming.
[0] https://www.amazon.com/gp/aw/d/B07F17ZHJY
[1] https://www.amazon.com/gp/aw/d/B079736Z53