You pack more and more energy into a smaller and smaller space, something has to give. In this case, the result is that until the "next" generation of battery comes along where the safety problems are resolved, we need caution lest the energy comes out sideways and all at once, doing what chemical energy does, degrading itself down to heat. Or heat and mess.
But lead acid had its own hazards, mostly from the contents themselves, being lead (toxic) and sulphuric acid (extremely corrosive) but also from the construction. Plates that could warp under heavy current and plate to riser joints that could set the hydrogen/oxygem mixture off. NiCad stuff was disinctly unfriendly, not sure what lurks inside NiMH batteries, but I bet it ain't health-giving.