The Casio CTK and WK models are often criticized for the quality of their sounds, and having owned a CTK-710 I can certainly agree that most of its sounds were (in my opinion) inferior to the equivalent Yamaha sounds-- although the Casio did have a number of voices which I thought were quite good, including its default Acoustic Grand Piano sound.
And I've heard people say that the Casio sounds are improved by adding DSP effects and playing the keyboard through some good speakers or monitors. The same can be said of Yamaha's voices, and no doubt of any other brand-- sometimes a good deal of tweaking is required to get a sound that you're satisfied with.
I've heard some people say that Roland's voices are better-sounding than Yamaha's, and others say the opposite, often depending on which types of voices are being discussed-- acoustic piano, electric piano, drawbar organ, violin, etc.
Anyway, it's certainly true that the various brands have their own identifiable sounds, and I think that in the long run it's not so much that one brand sounds "better" or "worse" than another, but that they sound different than each other, and different people are going to form different opinions as to which brand's sounds they prefer. It's all very subjective, as can be seen when different keyboardists argue back and forth about which acoustic piano voice is the best, or which virtual piano software is the best-- each keyboardist has his or her personal favorite, and if you listen to several people talking about a specific voice or virtual instrument you might think they were talking about different ones!