The series resistor charger may probably be the nicest kind of charger for the battery. It will start with an allowed charge current, and then decrease when you get a voltage rise.

The problem is that the voltage in the cell is varying so little, so the charge current must be low with a series-resistor charger. Most people wants faster chargers.

Constant chargers can charge faster - but if just constant charge, then they will either charge very slowly or will overcharge the battery. Generally 1/10C will be considered ok to charge constantly. Some cells can stand this. Some can't. Most people can not stand 14 hours to get a full charge.

A problem with maintainance mode is that different cells needs different amount of current to stay charged.

Many chargers want to charge a bit faster than 1/10, but not waste money on measuring temperature and catching the voltage bump when the battery is charged. Instead they just use a timer. This leads to over-charging if a non-empty battery is charged. And it leads to under-charging if a high-capacity cell is charged. And over-charging if the user restarts the charge cycle to get their high-capacity cells topped up.

And chargers that do detect the voltage bump will only detect it if the battery was not fully charged when put in the charger. If a fully charged battery is put in the charger, then only a timer or temperature supervision can end the charge. And the temperature sensor will be a bit late, resulting in undue wear.

With slow chargers, all cells can be put in series. The weakest cell will get a bit of extra charge, but at so low current that it doesn't hurt it. But that requires a low charge current to let a weak battery survive. Or a good detection of a voltage bump from any cell and then leave the other cells not fully charged.

Many small problems, and the biggest of them all: You can normally not find a charger will all the intelligence and individual control for each cell that can be configured to not run in fast-charge mode. So in the end, there are two choices: Fast chargers and short battery life, or very slow chargers. The third choice: Smart charger with nice battery handling does not get any market shares since people do not want to pay for expansive and slow.

And that is for dedicated battery chargers for standard cells. Chargers for equipment with built-in batteries are a special case. Hardly anyone buys a tooth brush etc. based on the quality of the charger - only based on how long a single charge will last, and how fast it can be charged. So most such chargers will probably overcharge the battery. Either by always running a too high constant current, or always running a timer (even if the batteries was already full).

By the way: I can't see any indication that any NiMH battery I have is suffering from any memory effect. The only problem I have seen is that many cheap chargers do not fill them up, since they turn off on timer - and the timer is intended for smaller-capacity cells. This can lead to the individual charge in a set of batteries to diverge more and more until you need to discharge all cells to a known state - or put them in a more intelligent charger that fills them fully. NiCd cells to have the dreaded "memory effect", but it normally happens if each charge/discharge cycle is of identical length. Many problems with NiCd is that they diverge because the charger isn't filling all cells full and people then think it is the memory effect that they see.

And the recommendation to charge a new battery full and then run it fully empty before next charge is normally to let supervision chips or microcontrollers learn the battery capacity to later be able to guestimate the remaining capacity. With flat discharge curve, it isn't possible to just measure the voltage to figure out the remaining charge.