Its pretty simple, to get the same performance with nicad you need 3 Nicads since 1 A123 cell is 3.3V. Therfore much heavier. Good Nicads can however deliver more in short bursts than A123s can. But still, thats not a safety issue, that is up to the roboteer if that is sufficient.

I run Hannibalito 3 on 4 A123 cells in series (13.1V) with 2 Speed 900s for drive and I can do about 2 fights with them on 1 charge. So more than enough for 1 fight, and I have no complaint about performence. and for a 350 gram batterypack thats nod bad at all.