Realizing you can't have your cake and eat it too, are any of these things good at being both a hand warmer and power bank.

I had one. No, it was no good at warming. Even on high, you could barely tell it was on. The most pitiful excuse for a heater that I ever encountered. Then they recalled it, because it was a fire hazard. Their recall said to immediately stop using it for any purpose (I had already done that, long before).

YMMV with a different model/brand.

Timely question. I am about to gift wrap and place one under the tree for Mrs. Hikermor, who often takes a hot wter bottle to bed to keep her feet warm. It is touted as a power bank as well, and of course, incorporates a flashlight (Don't they all??)

I'll let you know how this one works.

Try to stick with a name-brand one if those exist. Mine was a no-name one, probably a Chinese import, sold by an American retailer.

When mine was recalled, I filled in the web form and they sent me a special latching shipping box, I suppose to handle the hazard of the burning battery. That shipping box was cool, and probably worth more than the heater. It was like a high end waterproof Otter box or Pelican case. So I ignored the recall, kept the case, and dropped the heater off at a battery recycling place. I was happy in the end - not with the heater, ... with the shipping case!

My surprise Xmas gift is branded Celestron, a name I believe I have heard before. It comes in a fairly typical container. We'll see....

I know of them for making telescopes, astronomy gear and binoculars. Celestron been around for many decades, and their stuff is good. I have some of their telescopes and optical eyepieces myself. I looked at their website and they do indeed make LiION power banks (to power their telescope electronics) so it's probably the same company that sells your heater/power bank. I would trust them to make something much better than my cheap Chinese junk heater. A heater would be a good idea for astronomy - I remember freezing my butt off more than once back when I was actively in the hobby. Standing around at 3:00am in the dark, in the Colorado mountains, without moving much is cold - even in the summer!

Bought a Dryguy brand to try. Will report
(Note: they say to charge the battery every three months - so at least once in the middle of Summer.)

Two month report, Dryguy works well, holds a nice charge and is warm but not too warm. Recommended.

We've used these all winter. Pretty solid. USB-C.

https://www.amazon.com/gp/product/B07GX9T1WX