About the OP's question about balancing the cells. 12volts nominal at 3 volts per cell is only 4 cells in series. It is not quite like other applications were you have up to 100 cells in series operating at full current
capacity, (IE electric
car). If the cells go out of balance, (less likely with matched sets), you will notice an imbalance in resting voltage after a full charge as the first symton, and reduced capacity as a second.
as a set, and periodically monitoring individual cell voltage after charging
, (and after discharging), will most likely give you plenty of advance warning that cells need to be rebalanced. And advantage you will not have with a SLA due to inability to get at individuals cells in a trojan, etc...
Other than smaller size and weight for same capacity, and slightly more care when charging, and discharging than Lead acid, (if you want to protect your investment)., there are really no disadvantages to Li compared to lead.
The only changes I noticed in switching to Li in portable power tools, and my laptop
is much longer use times between charging, (and slightly more heat).
Two things I have heard from users of Thundersky batteries. 1. Sandwich batteries in a metal case to prevent swelling from heat as it can cause them to lose capacity and fail prematurely, (this was from an electric
car owner that used them at 200amps or better).
2. Have forced air cooling
compartment, (also required for lead batteries in same application), see above.