The price is only one problem with one size fits all drop in 12v batteries. You have the same issues as lead only worse.
When one battery gets out of sync everything shuts down. They need to communicate between them to work together properly.
Each one has it’s own bms. Bms in series don’t play well together, as each 12v battery sees the full 72v under certain conditions. I’m sure the battery companies try hard to overcome this. But why?
Just build the proper 72v battery with a single bms. (battery management system)
AH= the amps a battery can deliver in one hour.
At 30mph a Gem can use 250 wh per mile.
That’s 3.5ah per mile.
a 100 ah battery / 3.5 = ~30 miles.
Very rough best case est. Subject to large losses in lead batteries and driving style.
Hard for me to go into too much detail without putting on my sales cap. My hobby after retirement is electric vehicles. Specifically batteries and high performance. I buy whatever I can find surplus and reconfigure to work in specific cars. Marking them up an obscene amount to cover my addiction and working pretty much full time on helping forum members.