Basically, the ideal PPI of mac displays is a multiple of 110 PPI. So, for retina quality you need a display of roughly 220 PPI, which is what you get from 5K at 27 inch. A 27 inch 4K display is around 160 PPI. If you use that in 2x mode, things will appear too large. If you set it to scaled mode to make things appear the proper size, there are display artifacts (like shimmering when scrolling). In fairness, it's not super obvious unless you know what to look for. But if you're already spending money on a high end screen, why should you have to compromise?
That seems pretty outdated information, given that the OOTH default Retina scaling in MacBooks have been non-2x fractional since 2016 (1400x900 for the 13-inch's 2560×1600, 1680×1050 for the 15-inch 2800x1800).
Basically, the ideal PPI of mac displays is a multiple of 110 PPI. So, for retina quality you need a display of roughly 220 PPI, which is what you get from 5K at 27 inch. A 27 inch 4K display is around 160 PPI. If you use that in 2x mode, things will appear too large. If you set it to scaled mode to make things appear the proper size, there are display artifacts (like shimmering when scrolling). In fairness, it's not super obvious unless you know what to look for. But if you're already spending money on a high end screen, why should you have to compromise?