Do you already have the Mini? If so, just use that and don’t waste money. If not… Nucleus has basically one advantage to it – it’s quite quiet (but then so is Mac Mini). If you will have it sitting in your listening room, it might be important. If you have it sitting somewhere else, it does not really matter.
Theoretically, Roon themselves would support the Nucleus, but software-wise they support Roon on any officially supported (ahem) OS anyway, and if you buy a new device you get manufacturer’s warranty for the hardware anyway.
From the usability point of view, while Nucleus is mostly plug and play, it (and a very similar homemade NUC running ROCK OS on it) have pretty much zero management and monitoring capabilities. And if it runs out of memory (e.g. you have too large a library) it simply crashes instead of slowing down.
Given that you can’t even get the Nucleus to gracefully shut down if the power goes out (obviously, you need a UPS connected to it, but why would you run a server without one anyway?). And hardware in a Nucleus is a totally generic PC in a nice case with decent thermal management.
Personally, I am running Roon on a Synology NAS sitting in the office. But then I need a NAS for other reasons. If you can find some use for it, might as well run in on a NAS and use it for something else at the same time. If you don’t, and you’d need to buy something, just get some $400 or so PC and shove it in the closet somewhere…
Here we could get into a long (and pointless) audiophile discussion about what “best” means. dCS definitely knows how to make a good DAC. They pretty much wrote a book on that. Alas, as far as I am concerned (or any blind test, too) at this point you get equally accurate output from a DAC that costs 1/100th of a dCS. It’s something you buy because it’s well-built, good looking, and has some impressive (if totally overkill) engineering in it. Kind of like buying an IWC watch – great kit, certainly lots of good reasons to get one, if the price is affordable, but you very much do not get it because it shows time better than a $30 Casio 
Can you try a Wiim or something through the same setup and at the same volume (not that any high-end dealer would agree to that!)? You might find yourself surprised…
I don’t have (or want) a 130, which on top of other Rose software problems also is finicky connecting to external DACs, especially third-party ones. With the other Roses I have, I have never had any issues using them as Roon endpoints. Annoying to have to check whether the latest update had wiped out audio output settings again, but so far (knock on wood
) that part has been quite stable. Using one of their units with a built-in DAC removes a lot of moving parts and points of failure from the equation, still gives you that nice big screen, and sounds just as fine. And costs less. If you create a good correction filter and use it in Roon you will even get quite state of the art sound quality in the actual room…