Shawn,
Thermal runaways can, and do still occur with Ni-cad's. The biggest cause is poor maintenance in the battery workshop. Something as simple as having a few mis-matched cells can start it happening.
As you say, lead-acid batteries are more reliable and cheaper(considerably). One of their main advantages is in maintenance time. A lead-acid can be removed, cap-checked and refitted the same day. If a Ni-cad buggers about during maintenance, they can take days to test. (and lots of money in replacement cells)
With the new lead acid batteries, there is not usually any requirement for these to be vented, as they are totally sealed, and any gasses produced are recombined into the cells.
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