As for the piano, there isn't that "signature" solo piano sound that would be simply live piano. In fact on the headphones there was no brightness to the piano, but once plugged in PA, the piano was there, just not the sweet brilliance of solo piano. The PSR has more refined piano sound.
But many instruments are pretty good on BK-5. I like the brass and saxes, there is something not so sterile (as with PSR) on the Roland with them and they sound pretty live. Also Roland is not shy with dirty sounds on various synths, organs or even electric guitars. Again, Yamaha would be a bit sterile there. With roland many sounds like synth or organs would have for example that bit of tiny distortion in the tail - a small but nice touch.
The strings were nothing special (but I play real strings, so I am not very easily impressed by most keyboard strings, neither on the PSR, they all lack dynamic depth - but for example the kurzweil PC3 strings do sound good). And instruments like violin don't sound like violin at all on neither.
I remember I played some wild random styles on the BK-5 and got a banjo sound by accident. Well this was maybe the first passable banjo I heard for long time...

What bugs me a bit about PSR is that the instrument in styles do not necessary blend, but are sort of standing sharply.
I could hear much more "blending" in the roland on many of the styles and sounds. Also the orchestration seems bit less flowery and more real than in yamahas.

But then.... well, the PSR910 does have far much "fun" included (well it does cost 70% more). The yamaha is more of this decade board (with all the gadgets, tricks and whats not) while the BK-5 sort of feels like a bit last decade board. There is no sequencer but there is a style pattern editor unlike the bk-7m, but its usability is dubious.

At the end I returned the BK-5, but I think the module version may be probably better suited for other keyboard owners. Once you use BK-5 as module, things do not sound that great and it become far more fiddly and too much button pushing. The BK-7 use external keyboard to trigger the layered sounds and accompaniment and I couldn't make BK-5 behave that way (it behaved like a normal 1-16 module and for that money there are better modules)

And last thing, playing midi, I liked the PSR rendering of GM and XG bit more. The roland would need constant tweaking of sounds, while the PSR usually work reasonably ok straight from the file.
Anyway, it is $999 board and there is plenty of that in it but one would always think - why they didn't charge hundred more and add this or that.... honestly, at least sequencer.