Definitely, and it's probably not very noticeable, but there's no escaping the physical problem of not a perfect 1:1 or 2:1 pixel ratio.
Say you have a UI element that's supposed to be 9px wide. If you have perfect 2:1 pixel ratio you can use 18 physical pixels and it's as sharp as the monitor can be. If you're using UI scaling of say 1.5, the monitor needs to use 13.5 physical pixels to render it. Except that's not possible, so it will average together the 13th pixel with the 15th pixel (simplifying). It's basically introducing an extra anti-aliasing pass to your frame. This is is why the Apple/LG 24" is 4k and the 27" is 5k. All that said, I'm sure 4k @ 27" even with 1.5x scaling still looks great and you'd never notice the difference, except if you had the two right next to each other.
I think this always applies. For instance the border on this textarea is 1px. There's nothing the OS can do, it's a physical issue. If UI scaling is 1.5, the correct answer for the size of the border is now 1.5 physical pixels. The best you can do is color the first pixel black and the second pixel the average of the first and third pixels. I might be wrong on this but I can't imagine what else it could do (other than mis-sizing the element).
I do agree that it probably looks fine and you wouldn't actually notice, unless you had an @2x monitor right next to it (of which there are sadly very few options)
I don't think that's possible? Say if you had a perfect 1px wide black line @1x and then you change UI scaling set to 1.5, you now have a 1.5px (physical) wide line. Which means you'll get blurring.
I suppose if you put it quite far away, a 27" 4k @2x scaling would work well. It'd feel like a 24" 1080p monitor with retina sharpness.
For all the folks confused why I'm saying you don't want a 27" 4k monitor for text, it's because the PPI isn't nicely sized for 1x or 2x, which means you have to set it to scaling that rounds and blurs subpixels.
By the way, I'm sure most 27" 4k monitors look fantastic and you should enjoy them! Especially if you don't mind or notice the scaling rounding or if you run it @2x and like the 1080p real estate.
If you go to true 2x scaling, things will be the same size as 1920x1080@1x on a 27". For me, that's HUGE and you don't get enough screen real estate for a 27". If you use 1.5x scaling you will lose sharpness because things won't line up to pixels perfectly.
For text rendering, 4k doesn't really work for 27" or bigger. It'd work for 24" because you could do true @2x, but there aren't many out there. For 27", besides the UltraFine 5k, I'd prefer just using @1x with 1440p too.