The review reflects a general rule about monitor resolution; given the same resolution, a 27" monitor will be slightly less clear on text and fine lines than a 24" monitor. This is where pixel density matters. Unfortunately, this isn't measured the same way a monitor's screen is measured, as pixel density is about how many pixels there are per a standard unit of measure, but monitor size is measured by the total screen length from the lower left corner to the upper right. Just like that one South Park episode, they measure at an angle to get a higher number.
1920*1200 = 2,304,000 pixels
3840*2160 = 8,294,400 pixels
Both of these resolutions have a 16:10 aspect ratio. We now have all of the numbers we need to determine quite a lot about the monitor. We have the hypotenuse of a right triangle (24") but we have a different standard of measure for the sides (1920px L, 1200px W). We can use the Pythagorean Theorem to determine what '24"' means.
1920sq + 1200sq = 3686400 + 1440000 = 5126400
sqrt of 5126400 = 2264.155471693585
3840*2160 = 14745600 * 4665600 = 19411200
sqrt of 19411200 = 4405.814340164597
Now we know how many pixels lie across that hypoteneuse for each monitor at the given resolutions. This lets us determine an estimate of density by ratio.
2264.155471693585 / 24" = 94.33981132056604 approx ppi diagonally.
4405.814340164597 / 27" = 163.1783088949851 approx ppi diagonally.
Clearly, the higher resolution monitor here wins out. That's because the 4k monitor is nearly double the resolution, and is not double the size. The user who left the review likely had trouble seeing some things clearly on the 4k monitor because they were shrunk in size as a result, and lowered their resolution to 1080p; 1080p is not the native resolution on the monitor, and will appear blurry by comparison. This will also happen in fullscreen apps which do not support higher than 1080p.
For text, I'd still use the smaller monitor. Given what I just said about reducing resolution of your display, text will appear smaller in native resolutions at 4k than it does in 1080p at the same font size and kerning.