Well, there are a lot of popular things that I'd argue aren't much good. I could name cars, bands, equipment, whatever.
The other thing to consider here is that numbers don't always tell you everything. I'm a fan of measurements, its the only truly objective way we have to compare stuff, but the fact is you can have a very pleasant sounding amp that measures pretty badly. A lot of people love Magnavox console amps, but in stock form quite frankly they're not awesome on paper. Conversely for years I hated the sound of my Phase Linear amp. Measures great, meets all its specs, but it made me want to plug my ears. I changed some parts and its much better now, still measures about the same but the sound is not the same. I can't quantify why its better, but it is.
Also, just because its a simple design doesn't mean there can't be "gotcha" elements to it. Layout, component quality, lots of other little things go into it. I've had far more problems getting basic 5 tube AM radios to work right than I have with multi-tube complicated shortwave receivers. The simple stuff sometimes has to be precisely just so in order to work, where the complicated stuff can be somewhat immune to less than perfect component placement and such.
The ultimate test is the listening test, no doubt about it. But with 4 amp (this cheap amp, Acurus, PA-7 Stasis and two of my amps) tested in detail both FFT and listening test , also with 3 pairs of speakers ( JM LAB, KEF and Monitor Audio). I do start to see a pattern. To be honest, all amps sound very similar, particular the SS amps. I truly can say the sound signatures are only slightly different. It's not like one make the human voice a lot better, or this better or base better etc. The tube kit amp is definitely a little more mellow and the high is not as pronounced. But that's mainly because it roll off 3dB at 20KHz where all the other SS amp are flat to 20KHz.
The major difference is the sound stage, the 3D effect, the fullness and separation. There is definitely a correlation between THD and how good these are. I have two members here at the beginning of this thread think I gone to twilight zoo when I start talking about this, and that the speaker cable is the key to make an amp stand out ( or bury the amp). But I am sure you've seen my threads on these step by step experiment and demonstrate this.
This is what I found from testing all the amps I have, the lower the THD, the better the soundstage, separation, 3D, fullness. Even before I added the cathode follower, the sound of the kit amp just stay on a flat wall in front of me rather than coming out and come alive. It's the cathode follower that saved the amp from going to the trash. That cathode follower lower the THD by half and the sound just starting to come out of the wall.
I want to stress, all my amps and speakers are NOT low quality, even the cheapest Acurus is nothing to sneeze at compare to most of the Onkyo, Pioneer, Sansui, Kenwood, Marantz and the like, definitely a class higher. My bad pair of speaker is KEF, it is by no means "lousy" speakers. I still paid over $600 at the time. I put a lot of effort in doing this.
I am confident the layout of the tube amp is as good as it can, I spent a lot of time thinking about the layout wiring before I even start soldering. I turn the tubes and point to different directions to make the wiring smooth and avoid crossover. I trim to minimum length to optimize the layout. So the poor quality of the kit amp is not due to layout. This is something I am very good at.
Too bad we all live far away, all we can do is talk. One listen speaks a thousand words, but we cannot.