It seems much more efficient with a small, DC, li-ion UPS than an external UPS which will use an AC inverter (and now you gotta decide, spring more for pure sine wave?) and heavy lead-acid batteries.

It seems like a gimmick to me. It depends on the use case, I guess. So you have your NAS on a built-in UPS. Now what about the rest of your network? Switch? Router? Modem? You still need an external UPS.

It's mostly a gimmick, but for home/small business users I think it is actually pretty useful.

A mobile power bank-sized battery like this can probably power a NAS like this for at least 10-15 minutes (personal experience messing around with USB-C).

Most home/small business NAS usage is SMB file sharing, and SMB writes are async. Just a minute to sync writes and close the file system safely is huge for most users.

As someone who has supported a small business, just being able to handle the 5 minutes between someone running the coffee maker at the same time as the fridge and then resetting the breaker is huge.