In several forums I've often read about e.g. Yamaha sounds having a higher quality than e.g. some Roland sounds etc.

I think two different things are sometimes mixed up: the technical audio quality of a sound on the one hand and the realism of a sound in relation to the 'original' acoustic instrument on the other hand.

An example would be the following: If I just audio-recorded a real French Horn, it would be 100% realistic, as the recording cannot render anything else but the waves from the real French Horn captured, of course. But if the recording was made in very bad MP3 quality, or even taken from an AM radio station, its sound quality would be awful, but still render a real French Horn.
On the other hand, the sample of a French Horn can be very unrealistic, taking only a short attack phase and looping it or making other mistakes in the sampling process (wrongly placed mics etc.), but if it is implemented in high resolution etc., its audio quality can be fantastic. Still, it would never sound like a real French Horn.

In that sense, I think many 'old' Roland samples are realistic, but lack audio brilliance, whereas some Yamaha samples have polished audio quality, but lack realism.