It used to be that lithium ion and lipo batteries were very prone to catching fire during charging due to not having a balance plug and a charger capable of balance charging. These types of batteries are critical in terms of the max charging condition they allow, and so lacking balance chargers and balance plugs on the batteries if one cell got over charged things could go very bad. These days they pretty much all have balance plugs and the chargers have balance ports for them. However, that still doesn't mean you shouldn't be careful and always charge in the middle of a cement floor and where you can keep an eye on things. As a general rule, lipos can be charged at 1C (1 hour charge) and that isn't pushing them like nimhs or nicads can, and the lipo won't get hot either at that rate while nimhs or nicads can. There are some lipos now that can be charged at 2 or 3C but I don't recommend that. Not just for safety reasons but also so it isn't hard on the pack for longer life.
Really nimhs or nicads are at equal fire risk while charging and such. All pose a risk. The other main thing about lipos is that if you puncture the soft shell they have they can explode or catch on fire when the internals are exposed to oxygen. They should also NEVER be shorted, not even for a split second, as that can cause a reaction that makes them go nuclear too. All that being said, the lightness of lipos, the cheap price (if you know where to look) compared to nimhs even these days, and so on makes them my choice. You can't run them down past 3V per cell (9V total for a 3 cell series pack) or it might ruin the battery, so make sure whatever you use them in has a low battery alarm that kicks in before it drops below 9V. They don't self discharge on the shelf and are super light, so for those two reasons alone I like them. That said, I also store them in a metal box (with inside of box painted so no potential for shorts) just to be safe, and I keep them in the refrigerator as that prolongs their life for long term storage.
