The advantage of NiCads is that they give a constant voltage right up to the point that they are discharged whereas Lead Acid batteries give a drop in voltage as they discharge. NiCads are also a bit lighter. However, they have their problems.
One aircraft I flew with NiCads had one battery right above the pilot's feet. in the case of a runaway, you isolated the bat right away and got on the ground asap. We had a "Bat Temp" caption on the CWP.
The 747 classic has it's NiCad batteries on the flight deck!!!!!! And no warnings of an overtemp. The sit behind the flight engineer's panel My company had a nasty incident with a thermal runaway. Luckily, they got the aircraft on the gound in 25 minutes. Could have been a lot nastier if they were half way across an ocean. But the fumes were a rel problem.