I've been daily driving a 27Ah 52v battery (aprox 5kg) in my backpack for 3-4 years now. And from my experience:
-Back fatigue after looong rides (to be expected with any heavy-is backpack)
-battery does not run hot (or even warm)
-I fell a couple of times, everything is OK.
-mtb is hella-fun with a light and balanced bike. The backpack is not noticeable in the first 2 hours but needs to be firmly strapped to handle jumps.
-I feel the battery is better protected in my packpack on rough terrain and jumps rather than being on the frame.
-it's a pain to be tethered to the bike. But I've gotten used to adding a couple of steps to unplug when getting off. The cable extends enough so I can get off, rest the bike down and not pull on it.
-used a keycard extender thingy to auto-retract my cable when unplugging or extend if random tuggs occurred.
-not really worried about explosions, it's not a pouch battery, it's an 18650 pack and the ends are pointing up/down not directly at my back. I did a fire drill a couple of times and can safely chuck my backpack reasonably fast.
Other notes: I stupidly put a jacket with a metalic zipper in the battery compartment and it shorted the charging leads (mini xt60). Nothing exceptional happened, there was a pop, part of the connector melted and the bms shut the battery down. All was fixed after I replaced the connector. No damage to self. I could tell there was a problem and took it off immediately.
If anyone has any questions go ahead
End note: I'm looking to add a small frame mounted bottle style battery to use on really short trips (store / neighborhood) when I can't be bothered to take my backpack.