For any BMS that uses FETs (rather than contactor), the limitation is the voltage the FETs are rated for. You don't want to exceed that.
If you place packs in series, then as soon as a BMS tursn off, it's FETs now have the full voltage of the entire set of seriesed packs across them. If that exceeds the ratings of the FETs, they will probably fail. They might blow up and open the circuit, shutting down all power. But worse...they might fail shorted. That means that they will simply not be able to turn off, so the pack will continue to operate, even though the BMS had tried to turn it off to protect it from whatever condition caused it. That's usually something bad enough that letting the pack continue to operate could cause cell damage, and that could lead to a fire, at any time now or in the future, depending on what happens to the cells.
Even if nothing happens then, the BMS is still unable to ever turn off those FETs. So it can't stop discharge anymore, and something is likely to happen at some point that *will* damage the cells.
If you look up Fechter's posts about seriesed BMS / packs, he shows a method to use a diode in parallel with the packs to help prevent this kind of failure.
