I am wondering how terrible this app is if it needs it own device just to stream music ?
It's not an app, or at least the component (Roon Core) we're discussing that you need to install on a 'server' isn't an app in the traditional sense of an app on your phone.
The Roon Remote app (that you use to control playback) will happily run on any modern iOS/Android device or Mac/PC desktop.
Roon Core should (in theory) have no difficulty streaming from a low-end machine — as your say it's just audio streaming. But it only comes compiled for 64-bit x86 architectures and most low-cost NAS devices use lower end CPUs. I agree that is probably unnecessarily limiting, but not so different from needing a Mac if you want to run software only compiled for a Mac.
I expect this is somewhat by design, for a relatively expensive bit of software, sold to non-technical users, it makes sense to limit the platforms it runs on to hardware you know won't have issues. I imagine this is behind their thinking re. offering a turnkey OS (
ROCK) that can be installed on a relatively cheap NUC hardware (as a DIY alternative to their
propriety hardware) is that it ensures a standard platform across a large chunk of their user base that is easier to debug should users hit issues. Remember they're not aiming the product at people who like tinkering with servers or are looking for the lowest-cost/free option, rather the bulk of their users simply want to playback their local (and online) music collections (via a decent interface/app) and are willing to pay to achieve that if it works well.
In terms of processing, I found that one stream wasn't ever an issue on my (high-end for a NAS, low-end for a PC) 64-bit x86 NAS. But if the NAS was busy with other tasks and I streamed different music to different endpoints simultaneously and especially if I enabled DSP processing on each stream (a similar issue to transcoding with Plex on a NAS) then the CPU usage grew and I occasional got dropouts. Indexing also requires more CPU processing - particularly analysing audio files for volume levelling, although in practice that's only an issue when initially scanning your local library the first time as most people won't be adding music in significant quantities after that.
But the main reason for giving Roon Core it's own server is that it i) simplifies setup/maintenance/support
(Roon will likely be quicker to debug issues for you if you are using a known/standard platform) and ii) eliminates the risk of it being affected by other processor or network heavy tasks also running alongside it.
Sure, you could configure your server to ring fence resources, but for a non-technical user it's probably easier and simpler to give Roon Core it's own NUC (running ROCK). It also makes it an effectively maintenance free device/setup.
But I think the key thing here is the OP said he was
non-technical and
looking for a solution to run Roon. Given those requirements, running ROCK on a used NUC (that can be picked up for ~£150) is the route most likely to fulfill these requirements, with the least hassle.