TellMeWhy, welcome to the forum.
Many try to get a flat lie but suspending the hammock tightly. Flatness is achieved by having enough sag in the hammock so that you can turn a bit on a diagonal with the hammock center line. Another issue is assuming they must sleep in the same position as they do in a bed. A bed has different support and pressure points than a hammock. It is not unusual for the body to need a night or two to learn how to relax in a hammock.
TellMeWhy, paradoxically to your question, Comfort is the biggest reason people move to a hammock (and no more crawling around on the ground). There's on Thru-Hiker on YouTube who is an avid single-wall tent camper. Although she often misrepresents hammock issues because she has no training, she also admitted that the hammock gave her the most comfortable night's sleep.
Here's the problem: setting up a hammock for camping/overnight sleeping requires some knowledge. If a person buys a short, cheap hammock off of Amazon or a "Big Box" store and strings it up tight, their experience will be ... less than optimal. But then they say, a hammock solution is too complicated because they have to learn stuff. They've forgotten that they also had to learn things for tent camping. They've been tent camping for so long that the little decisions they make are automatic. It's the same for a hammock. At first, finding trees the right size and distance apart seems like a chore. But after a short while, you can just look and see what you need.
Once you've determined the right sag for you (end-to-end distance is usually about 83% of the hammock's physical length), you can connect a line between the two hammock ends (if the hammock has a built-in bug net it already has this ridgeline) so it the future, you just hang the hammock so that ridgeline is just snug and you'll have that same preferred sag.
Of course, any one thing, hammock or tent, isn't for everyone. But without knowing how well-versed the person was in setting up the hammock and if they are new to hammocks, and given it a night or two, then one wonders about the "it's not comfortable" claim. A tent is not comfortable if they didn't clear the rocks, small branches, and pine cones from the ground under it. A tent is not comfortable if the ground slope is not right.
Check out, the book, "The Ultimate Hang". It will describe many details about hammock camping. Then see how your brother sets up his hammock. The hammock should be about 10.5 to 11 ft long. When hung, the ends should be about 9 feet apart. When he sits in it, it should be suspended so that his feet are solid on the ground and he can just stand up (with one hand on the front edge and one on the back edge). And when in the hammock, he should turn a bit so his head is on one side of the hammock center line and his feet are on the other side, a slight diagonal.
One other wrinkle is that some people prefer a bridge hammock. That style (look up Ridge Runner or Banyon) uses spreader bars at the foot and head ends and has a different feel than a gathered-end hammock (GE).