I had a bunch of comments -
You're really down on serial charging. Both parallel and serial require keen attention to proper connections - but I think there's a big advantage to serial when using a proper configuration (typically up to 3 + 3 since most chargers are up to 6S) - that the charger sees each cell voltage so the depth of discharge issue isn't a problem. Is that right?
with series charging you need a high watt charer or you will have a low charge rate. if you have a 50w charger the charge rate on 6s will be around 2.3amps if you are charging 5000mah it will take over 2 hours . if you have a 400w charger the charge rate on 6s will be around 18amps if you are running 5c charge rate packs if you are charging a 5000mah it will take about 20min.
!!! I AM NOT SAYING YOU SHOULD POUND YOUR PACKS AT 5c ALL THE TIME!!!!
While the rate is a consideration for charging in general, it is not affected by choosing to charge serial or parallel. if you have a pair of 5A 3S packs in parallel, a 5c rate would be 50A, 11.1V, demanding 555W from the charger. The same in series would be 25A, 22.2V, demanding 555W. In that situation I choose to charge in series because each cell voltage is seen individually.
with parallel charging you also need a high watt/amp charger or you will have a low charge rate. say you have a 7a/50w and you are trying to charge 2 x 3s 5000mah= 10,000mah the charge rate will be around 4.6a=over 2 hours charge time. but if you have a cell in one pack go bad you wont know it because the other will mask it