Not all electronics will work well on nimh batteries.
A nimh battery has about 1,4V if it comes out of the charger, but that voltage will drop very fast to about 1,2V.
That voltage (1,2V) will stay almost the same untill the battery is “empty”.
An alkaline battery starts at about 1,6V, and the voltage drops gradualy during use.
Some electronics will stop working at a battery voltage of 1,3V per cell, in that case it is not possible to use nimh batteries.
If there are 4 batteries in a device, then it starts at 6,4V with alkaline batteries, with nimh batteries the device will only get 4,8V.
Measure the voltage of the cells whent the device gives a low battery warning (or when it stops working).
If the nimh batteries are 1,2v or higher, then it is not possible to use that type of battery in that device.
Then you will need alkaline batteries, and the device will stop working after it used up about 2/3 of the useable capacity of the battery (you can use them in other devices that don’t have problems with a lower voltage, but they will not work in high power led flashlights…).
The lower voltage of a nimh cell can be a problem for some devices, but other devices will work better on nimh, because it is possible to get higher currents out of them.
A camera flash works much better on nimh than on alkaline batteries (shorter recharge time of the flash capacitor).