Hi Dave,
Apart from the points eloquently made by the earlier posters, of major significance is that rather than utilising and adapting a Raspberry Pi and software designed for general computing and educational use - by adding a 'piggy-backed' PilotAware 'Bridge' and separate dongles for 1090MHz receive, GPS, etc., FX was designed and built from scratch 'in-house' as an Electronic Conspicuity Device on a single, purpose-designed mother-board.
This of course meant that we were not confined to the functionality of previous PilotAware models or the limitations of the Raspberry Pi, but could develop the system (in functionality, efficiency and with a view to future development) as we saw fit.
As a result, FX is slimmer, lighter and more robust than its predecessors, with more efficient software, more efficient and reliable radio equipment and a much more friendly and easy to use Graphical User Interface.
It was also designed to be directly compatible with existing Flarm installations, allowing Flarm Users to easily add the full PilotAware functionality (including 1090MHz and P3i receive and the full range of ground-station and iGRID uplink) to their installations.
The board now has 2-way WiFi built-in - enabling it to pass data to EFBs and Traffic displays and at the same time connect to an external 4G/5G hotspot (such as a mobile phone) to allow it to interact directly and securely with the PilotAware Network via your own cellular network (this is also now much easier to set up than with previous models). This function (known as iGRID) provides a reliable '3rd pathway' (in addition to Direct Plane-to-Plane Communication and local uplink from our ATOM-GRID Network) for exchanging traffic, METARS, Rainfall Radar, Flarm, Fanet+ and Mode-S/3D, etc. data in areas not covered by the PilotAware ATOM-GRID network. It also significantly improves the effectiveness of the PilotAware SkyGRID relay system and allows automatic monitoring of system efficiency and uploading of future software updates. It can even upload and install your new licence key for you if required.
Comparison with earlier PilotAware models indicates that the inbuilt receivers in FX are significantly more sensitive and reliable than in previous Raspberry Pi and SDR based units, giving much better operational range for direct detection, or for accessing the local ground station network.
Ultimately, all of this means that FX runs far more efficiently, drawing significantly less power than either of its predecessors (around 150 milliamps as opposed to up to 2 Amps). Whilst this makes it particularly suitable for use in gliders or other non-powered aircraft (it can run all day long and then some on a relatively small battery pack), the fact that there are three separate powering options is also a distinct advantage when it comes to installing it in most powered aircraft.
I hope this helps clarify the position and provides further food for thought.
Best Regards
Peter