I have the Acer XR382CQK which is a 37.5” 3840x1600 monitor that has a 75Hz refresh rate and 21:9 aspect ratio with FreeSync support. I think it’s an excellent buy, especially when compared to the identical LG38UC99 with which it shares a panel.
Personally I think 4K gaming monitors in general are a waste of money, if you want high refresh rates of 100Hz+ then the technology is not there yet from a GPU perspective. Given the TITAN V struggles to give 60Hz+ in Ultra quality settings on AAA titles you’ll start to understand why!
Also the recently released 4K 144Hz HDR monitors cannot actually support the HDR10 standard beyond 120Hz without using chroma sub-sampling, basically not using a full 10-bit colour input, you’ll start to understand why the technology just isn’t there yet.
There are some good reviews by Linus Tech Tips amongst others that go into a lot of detail on this. The real sweet spot is 1440p ultrawide gaming right now, but even this requires a really good GPU.