If you're using a non-evenly scaled resolution (i.e. not exactly 1x or 2x rendering) it's not gonna be fantastic. Running at 'default' 2x my 2018 runs 2x 24" 4Ks without issue - the problem most people have I think is that they buy much larger displays and then want to use a higher rendered resolution, which is where the iGPU will struggle.
I'm not going to buy a 4k screen with the intention of running it at 1080p. There are plenty of large 1080p screens out there.
If I buy a 4k screen it's because I want to run it at 4k.
This doesn't mean I'm running it at a lower resolution - behind the scenes, the mac is actually rendering the display at 6016x3384 and then smoothly scaling it down to the native 4k of 3840x2160.
Because it's having to draw everything at a higher resolution than the actual screen, performance does take a hit, but I find everything still works perfectly smoothly and responsive (just don't try playing games at this resolution on the mac mini!)
I have a second monitor which is not high-DPI but 2560x1440, connected with a USB-C => displayport cable. Surprisingly, the mix of high-DPI and normal DPI monitors works really well, with no problems, even when moving windows from one display to the other.
It isn't running at 1080p. It's doing "@2x" pixel rendering (commonly called "Retina") - so it uses 4 physical pixels to render one "screen" pixel, giving you much crisper.. everything.
Though I get why you might think it's pixelated from stephenr's description.
He is not talking about not running 4k. He is talking about running more than 4k framebuffer scaled down to 4k (the "more space" option in Display control panel).