I have tablets in my home, with batteries (as fallback in case of power loss - my HA instance and router/APs are on UPS).

I’m using older (thus cheaper) Samsung tablets. Fully Kiosk works well, but recently I’ve started adding kiosk features to the HA Android app (no public PRs yet so it’s a few months away) to add some features (mostly sleep/wake via audio/camera motion, sleep screens, default pages for the specific instances etc.), so I can ditch FK.

Battery bloat is an issue - my Tab S6 is on its third battery (though admittedly it is nearly 7 years old now, got it January 2020), and my Tab S5e lasted 3.5 years until its battery started getting spicy. I’ve switched both to LineageOS to reduce heating + have more control over charging/discharging, and that so far seems to have helped. Unfortunately Samsung tablets don’t support running off of USB without a battery (they bootloop if no cell is detected), so pulling the battery and running them off a wall outlet isn’t really an option.

To me, a Pi or similar SBC with a touchscreen just seemed like a very hacky solution. I rent so I can’t add a box in the wall for the board and PSU, and tablets are just more elegant. Throw in a 3D printed enclosure/mount, and you’re golden.

I use mine regularly, especially since they wake whenever I walk past them.

E-ink, unless you’re writing your own frontend, I’d recommend against. There isn’t really a good eink theme for HA at the moment, and most panels that are good sized (10-13-16") cost a fortune (£250-300 just for the display, and you’d still need the brains). Meanwhile older tablets in good condition go for £70-150, and you get the full kit for that price, plus generally, brilliant displays (just limit your search to AMOLED tablets).