OK from the beginning:
At the most basic level, the software on the Nucleus is divided into two functions:
- RoonOs - a minimal operating system based on linux
- Roon Server - the application that runs on top of the OS
The update that you are being offered only updates the RoonOs part (from build 259 to build 261). The Roon Server application will not be altered and will continue to report the same build number (1151)
The version 1.8 or 2.0 versions that you see talked about in relation to ‘Legacy’ or current versions of Roon relate only to the Roon Server application installed on top of RoonOS. The version of RoonOs itself has no impact on functionality unless you have a modern hardware platform (Titan or later version of the NUCs).
You don’t say what Nucleus version you have - but unless you have a Nucleus Titan then there are unlikely to be any observable differences in upgrading to the latest build (261) of RoonOs.
The talk of ‘HDMI’ on thunderbolt/USB C relates to the Titan and later NUCs which have a Thunderbolt 4 port. This port, which looks like a USB C port supports USB rates up to to 40Mbps but it also supports (with the correct cable) direct attachment to an HDMI monitor (or in the case of Roon, an HDMI audio device). Until this last release of RoonOs (and the early access release that immediately preceeded it), HDMI over thunderbolt was not supported by the installed RoonOs drivers. Build 261 of RoonOS adds support for this functionality.
Having said that, I would be inclined to think of RoonOS updates (which happen very infrequently - the previous update was nearly a year ago) as the equivalent of Windows or OSX updates. Viewed in this way, I would suggest that it is a good idea to install this update even though you will not see any direct advantages or new features.
Quite separate from this, the primary reason for the existence of Legacy Roon Server (version 1.8 build 1151) was to address the initial need to have an always on connection with Roon 2.0. This requirement has now been removed and the latest versions of Roon 2.0 (from build 2.0.28 onwards) will work offline in the same way as Roon Legacy. This removes the primary ‘raison d’etre’ for Roon Legacy (1.8) and so it is likely that this build will cease to be supported at some time in the future.
As a consequence, you may well want to consider migrating from Roon Legacy (1.8) to the current version of Roon (2.0.38 has just been announced) in order to get ongoing support into the future as well as the new features: Folder Browsing, Playlist Hierarchies, Tidal MAX (HiRes FLAC) support, Playlist Improver, ARC etc etc