The only real downsides to continuing with NiCad or NiMH is that most have increased in capacity greatly from the old days. Which means longer charge times. And if you have a weight sensitive plane, finding small (lighter) packs can be hard to very hard.
Lithium Iron Phosphate (LiFe) are a great alternative to the older chemistries for flight packs. Nominal voltage per cell is 3.3v, so what is known as a "2S" puts out 6.6v. This is just a bit more than what 5 cell NiCads put out. Most systems will tolerate it very well, and give a bit more speed and torque to the servos. If pushing things makes you skittish, it is easy enough to find voltage regulators to put in line that will bring them down a bit, to be extra cautious. It sure is nice to be able to be able to routinely recharge in an hour (or less, depending on how low they are). Capacity is much higher, they are much lighter, and pretty safe from accidental fires. They must have a charger that is made to charge them. Almost all current multi-purpose charges do have the correct settings. While some do assemble their own packs from cells, it is better to buy pre-assembled.
A "3S" (9.9v) pack should substitute for 9.6v transmitter batteries too. But pack sizes and configurations that fit may be harder to find. And they generally have to be removed for charging, as the in-radio charging capabilities would no longer work for them.
Lithium Polymers (LiPO's) by themselves pack too much voltage for most old gear, They peak at 4.2v per cell hot off the charger. So a single cell is not enough, while a double cell is too much. Ditto for older TX's. They require their own charging routine, different than LiFe's. But again, recent chargers will have that as well.
The exception is an electric plane - they usually have an Electronic Speed Control for the motor that also supplies power to the rest of the plane, which has been regulated to 5v or so. So in those cases, LiPO's would be very useable for older equipment. With due care, they can be moderately safe, but standard recommendations are to always charge out of the plane/equipment, be monitored while charging, and be charged in an area where if something does happen they won't burn down your house, etc. Battery fires are actually pretty rare, but as Spiderman says "With great power comes great responsibility".
Slightly later editing: I should note, that LiFe's come off their high point after charge very quickly, and then put out their nominal voltage for almost the entire use of the pack. Then they fall off the cliff VERY fast. So using a meter to gauge remaining capacity is useless. Best is to fly a few typical flights for fixed amounts of time. Then see how many milliamps it takes to recharge. From there you can figure a rough idea of how many flights you can get out of a pack. It will be a lot! LiPO's have a bit of variation from start to finish, so a good meter can help you judge. But with electric planes one generally depletes the pack in one flight, so no need to try and guess "Do I have enough to go up again?" One almost never does,
Last edited by tedsander; 10-25-2021 at 08:35 PM.