NiMH batteries are generally pretty safe. They don't really require any onboard electronics, just a peak detecting charger if they're being fast-charged. If shorted, they may get *really* hot or ignite adjacent materials, but they're not as reactive as lithium based technology.
Li-Ion/Li-Poly cells are the dangerous ones. Basically they can reach a run-away condition very easily, and lithium is *very* reactive. They can ignite if punctured, charged too fast, over-charged, or if they drop below a certain voltage and are then charged at normal rates. They will also swell/burst if over-discharged or over-heated. Most Li-Poly/Li-Ion packs include a circuit with a thermal cutoff to prevent over-heating, a circuit to prevent shorting/excessive-discharge-rates, and a circuit that isolates the pack if it drops below a safe voltage (usually around 2.5-2.8 volts per cell).
I did purchase a pair of Li-poly batteries for a cell phone, which were no-name imports. They did not have any cutoff circuitry, and I suspect could have been nasty if they wandered out of spec (either through abuse or a defective phone). I used them with no problems though, but I think I'd reconsider, given what I know now about Lithium cells.
That said, the circuitry required for Lithium cells is becoming cheaper everyday (the functionality is avaiable in a single chip), so I wouldn't be surprised if "decent" third-party units do have the desired safety functionality.
D.