Charge density for NiMH is only 50% or so that of Li-ion, so presumably one would need 200 kg worth of battery... NiMH also has more self-discharge properties and would constantly need to be trickle-charged during any standby periods.

The engineers thought it would be within acceptable safety limits, and I'd bet with some modification they will ultimately be right, but it is still really scary to think about those batteries underneath the plane.

As somebody with an engineering background before medicine, took a nice look at the energy density:

100 kg of lithium battery would have about 15,000 W-h of capacity, or 54 megajoules. 54 million joules.

For reference, a stick of dynamite (200g) has about 1 megajoule.

It is energy equivalency of more than 50 sticks of dynamite... obviously, the ability to get this energy out quickly is somewhat more limited, but certain failure scenarios are concerning; have a look at this: (posted it before)

https://www.youtube.com/watch?v=pizFsY0yjss

Of course this is also concern for any electric vehicles in a crash. Time and experience will tell.