Not a problem, just expensive. Wires running to each individual would have to be larger than ones used just for sensing or small charge bleeding.
I was considering a design where there was a PCB on every cell with an individual charger, but it's just more cost and parts to break, plus no redundancy (unlike having two big chargers in parallel).
I don't think it's very expensive. Nearly all hobbyist RC drone batteries that are have 2 or more cells in series have a big discharge lead with all the cells in series, plus a smaller parallel "balance port" that provides access to each individual cell. Balance ports are used for charging, and can also be used in flight to wirelessly report the voltage of each cell. There are plenty of common (and apparently somewhat reliable) balancing chargers for under $30.
I’m not sure what you mean. I believe the problem is that battery cells wired in series will tend to discharge at slightly different rates due to slight differences in their composition from their manufacture, and that cells can be damaged when their voltage goes outside a certain minimum or maximum range, so it’s important to monitor each cell individually while discharging and charging.
Let's make a few 3-series batteries and charge them. When done charging we stack four of them together to get a single 12-series to power an e-bike. Would that allow for much faster charging?
I was considering a design where there was a PCB on every cell with an individual charger, but it's just more cost and parts to break, plus no redundancy (unlike having two big chargers in parallel).