Can a 3 cell LiPo be used safely in a transmitter that was designed for a 8 cell NiCad or NiMh pack?
It will operate fine from a 3 cell Lipo.
The main advantages of the LiPo are larger capacity and very low self discharge. The biggest danger is leaving the transmitter on and running it flat; been there done that. Another disadvantage is that you should remove it to charge it.
The low voltage alarm will be a bit low for a 3C LiPo; probably 8.5-9V. It may be possible to change the alarm voltage but that will depend on the transmitter.
I’ve been using a 3C 2650 for 2+years. Last year I did leave it on over night in the car and ran it flat (less than 3V), so I was back to NiMh for a few weeks. I charged, cycled and tested it thoroughly and it’s still going strong. It’s on for 1-2 hours a week and gets charged every 3 months, when it drops below 10.5V. Mostly on 35MHz, which uses more power than 2.4G.
35Mhz transmits more power (and wastes more power) with higher voltage from a fresh charge; the module gets hotter. 2.4G transmits the same power throughout the battery discharge but there will be a bit more heating inside the radio at the higher voltage.
There are videos and threads explaining a mod that significantly reduces the power loss in a 2.4G transmitter with a 3C LiPo making it last even longer. Lots of work and easier to charge it more often.