Typically battery backups are SLA (sealed lead acid) which is most likely why they don't get packaged with NAS systems - they're heavy, they're a dangerous good by transportation and shipping rules, they expand and can release gasses when under load and charged. The upside? They have way more capacity than that of a lithium-ion as shown in this little unit, they (probably) cost much less, and they last longer (5+ years vs 2-3). In terms of a NAS though, the capacity isn't really an issue if the only thing the NAS is doing is shutting down to avoid data loss/corruption.
I'm not sure if I've just missed it - but: How many drives of what brand and capacity, in what raid mode were used for the testing?
It would be good to single this information out into one of the section headings. It may be redundant as you presumably standardise this across NAS of a given bay capacity - but it would still be useful.
This is an impressive device. I'd definitively give it a try for home office solutions. The only downside I can see to it is that if the internal memory were to die (flash, ram), the backups are probably gone unless they keep using the same system/setting organization and it is forward compatible with other similar systems.
5 years is a long time and unless they intend to have the same product line around for a long time, it will either be EOL or replaced with an updated version.