the voltage drop of the weakest cell may be masked by the steadily increasing voltage of the other cells

I believe that many/most Li-Ion "smart" charging systems (and presumably the type incorporated in Karma) actually monitor the voltage of each cell in the pack. This allows the charger to be sure of not violating the fragile charge/discharge requirements of the Li-Ion battery type.

Of course, very few NiCd or NiMH packs are built with individual cell monitoring, and you are correct that many NiCd/NiMH chargers, even those with proper end-of-charge sensing, can overcharge one or more of the cells in a mis-matched pack.

The best one can do is not aggravate the problem by over discharging the pack, nor allowing a "dumb" charger to frequently/grossly overcharge the batteries.

SOME better NiCd/NiMH chargers can switch from rapid to trickle charging early (a little before the initial "fully charged" voltage sag would have occured), allowing a weaker cell to be fully charged, yet still top up the other cells using the mild trickle current, which if low enough can be safely disipated by the weak cell without further weaking it. That allows the stronger cells to eventually reach full charge.

Overall, you are correct that the weakest cell in the pack is the limiting factor, so the strategy must be to not aggravate that weakness further.

As a side note, some smart battery systems (and not just Li-ion) will recognize when a fairly full battery is reconnected to the charger, and won't actually allow the recharge, since the battery doesn't really need it, and the (unnecessary) recharge cycle would only serve to shorten overall pack life.