Hi @blaczero. It’s not so much that such a device IS a risk is that it MAY be because the manufacturer/quality/etc is unknown. There’s some good stuff out there and some that is, well, horrible. I think that’s what @Datahead was suggesting.
When we need such an item we make our own and typically use sealed gel cels. I showed one in another thread with a 7Ah battery but it could have just as well have been a commonly available 4Ah version — such as commonly used in alarm panels. The charger we’ve used most recently is this one and one similar to it. We’ve watched it’s use with bench instruments and have found no issues with it other than Vc never reaches optimum “cycle level” – which is not necessarily bad. But, YMMV.
You could also do as @mystery suggested and use a UPS. The down-side with that is they’re not efficient from a conversion perspective. The up-side is they’re commonly available & are likely UL/etc approved.
@MartinLangmaid (Slingshot 6 in UK) and his firm have made many of them as well, as well has @mldowling (Rising Connection in AU.) And, I’m betting other Partners and users have as well. Maybe others would care to comment.