Default rule
Do not put NiMH in a device unless the manufacturer allows rechargeable cells, or the specification clearly lists NiMH.
On BatteryByDevice, check the Rechargeable field and chemistry notes on the device page.
Why NiMH feels different
NiMH cells are about 1.2V nominal vs about 1.5V for alkaline. Many devices accept that; some do not.
Low-self-discharge (LSD) NiMH holds charge in storage better than older “high capacity” NiMH that go flat on the shelf.
Device settings
Handheld GPS and some instruments ask you to set Battery Type (alkaline / NiMH / lithium). Set it correctly so low-battery warnings make sense.
Alkaline-only devices
If the manufacturer says alkaline only (for example some aviation headsets), do not substitute NiMH because it physically fits.