Also, keep in mind that that airstone can get covered in coraline algae and might get clogged, so you'll have to replace it every
Personally I like the idea of a rechargeable battery bubbler. I have some of the same questions as the OP however it appears to me like some of these might use replaceable rechargeable D cells. If that’s the case, battery degradation isn’t a huge concern if they’re replaceable.
Not as an end-all-be-all backup, but something to just drop in the tanks on vacation. So the air stones getting covered in coraline etc. isn’t a huge issue for me. Just something to cover a 20-40hr power outage with automatic turn on so the fish sitter doesn’t have to deal with a generator for at least that long. From what I’ve found you have at least 4 hours before fish start to suffer, so aside from vacation or work travel, no need to leave them connected/in the tank 24/7.
The trouble is picking the right option. I haven’t decided yet.
I'm new to all this so I don't really know what a safe amount of time to go without aeration is... I have read after 2 hours things start to go down hill, but have also had a 15 year reefer says his was down for 29 hours with no issue once. If everyone thinks 8 hours would be safe I'd probably just use them for vacation, etc like you stated.