There are two reasons I think why this warning is true:
1. It is possible to engineer features into components, especially speakers, that will impress upon first hearing but become irritating over time. Exaggerated treble or bass will make a speaker stand out and seem more full of life but this becomes evident as a problem over time.
2. Some of the more important qualities of a component apparently really take time to notice and appreciate.
The first time I shopped for high end speakers, I did a quick a/b comparison among several, maybe five pairs. I was able to quickly eliminate one pair, the Gale 401-c, and focus on the others. But on subsequent visits to that store, I realized that the Gale was FAR more lifelike and nuanced than the other speakers, it seriously outclassed them. But these qualities just don't jump out at you, you need time to appreciate them. I bought them and got great joy out of them for over a decade.
And it was funny how friends would come to appreciate my stereo, really love to listen to music on my stereo, but rarely take serious note of it upon first hearing.