Size varies as like many other batteries, but most are Cylindrical with a small fraction that are flat designs.
The advantge is that they never "burn", thus they are safer.
Each cell is 3.3V base-minimum and have a similar discharge range as lipo.
Your AEG will not run well on 2S (2 cells) and thus you need a 3S. However that pushes to a power range that is just a hair below a 3S lipo so you need to prep your AEG for the higher voltage. This is the main reason why it has not caught on in the US.
Performance? It varies. Get a pack that discharges full power under a 30A load and you are good.
Due to the more complicated nature of the cell. Cost is on average 20-40% more over a LIPO of the same class.
LIPO are just more flexible. As long as you do not stab them with a nail, you will not have issues.