If you use designated chargers for them, they are. Most Li-Po battery packs are charged with a balancing charger and a smart charger, which detects the temperature, charge amps, charge voltage and others like battery type, cell count and so on.
Whileas normal ecigs, like Joy510 are designed to be charged with the chafger included with it, so both the circuits in the charger and ecig are designed for that specific purpose, so it is safe to charge batteries with the chargers that are designed to charge them.
If you try charging a Li-Po with a charger designed for Li-Mn or other chemistry, you will end up in trouble, so use only the chargers designed for the specific purpose.