Not really, unless your current Roon Core is CPU limited/constrained, to the point that you’re noticing audio dropouts or buffering, then moving your Room core to another platform like Nucleus should have zero effect on sound quality. It’s is after all just another computer running the Roon software, your DAC is the device handling the conversion of a digital signal to analog audio.
There will always be (real) edge cases where a particular DAC doesn’t work well with a particular USB implementation. Or situations where a particular setting on the machine or operating system running the Roon Core conflicts with a particular DAC or the audio path in some way — but those are edge cases and your likely to already be aware is something is wrong there (you’ll hear dropouts or glitches). I guess the Nucleus, being a known platform to the Roon team, should be more stable (with correct settings out of the box) in that respect — least because there is less for the end user to screw up.
Danny (and I assume the Roon team by extension) tend to (possibly sensibly) stay out of these kinds of discussions as there will always be people who say there hear a difference and if they do then I guess that’s fine. I’m certainly not going to spend my time arguing against them and I am sure Danny feels much the same. But it’s noticeable that the Roon team, in who’s interest it is to sell Nucleus devices, won’t make any solid claims re. sound quality other than to say that ‘it’s subjective’ (read into that what you will) and that the Nucleus offers improvements in other areas (true).
That said the Nucleus is a nice looking device (so aesthetically you may prefer it) and being fan less it could have an effect on your overall listening experience, simply because the lack of fans will reduce background noise if it sits within your listening room.
But in terms of buying one simply to ‘improve sound quality’ (aside from a reduction in fan noise) I’d tread there with heavy caution.