The majority of BMS's are just resistor dump balancer. The resistor dump balancers typically only pull 100mA to 200 mA during active balancing. In addition they only become active when a cell is greater then about 3.4v, near full charge.
This means if there is a moderate difference on SOC of cells and you are pumping a significant charge current into series connected cells the highest SOC cell will hit its high voltage limit before some of the other cells are charged up.
The best thing that happens is the BMS cell high voltage protection shuts down the BMS cutout switch preventing overcharging, But it still leaves you in a situation of a disconnected battery pack, Eventually the high SOC cell will bleed off by balancer and BMS will reconnect. The same process will continue to repeat until cells eventually get balanced. Not a great way to achieve balance. Without a BMS you can end up with an overcharged, bloated cell.
If you charge at a very low rate, below the resistor dump rate current, the pack will eventually balance out. At such a low charge rate it could take weeks to balance out large AH capacity cells.
Resistor dump balancers are really only for balancing out variance in cell internal leakage which is very small. If you have some capacity variation between cells and run battery pack over a wide SOC range the resistor dump balancers will not be able to keep up with balancing.