This is based on my limited knowledge, please let me know if there are any mistakes.
Coil spike generation with pulse motor has one benefit not exist in pure solid state implementation. Pulse motor has magnet that can aid the coil to produce more output during coil collapse event. But this need correct timing.
excellent illustration by JL Naudin:
Now imagine a situation bellow, where the coil south facing the magnetic north:
To be able to gain help from the magnet, the coil must be shutted off during the attraction wave.
Ideally, the coil must be shutted off before the attraction apex. So when the coil is shutted off, the electricicty from the magnet induction will add the coil collapse current.
The worst is when the coil is shutted off after the attraction wave, before the repulsion apex. So the electricity will reduce the coil collapse current.
A scenario for pulse motor that utilize coil voltage as trigger would be:
The voltage from the induction will start to turn on when the voltage is high enough. But the transistor will still be in linier / amplifier mode. The amount of current passing the coil will be proportional to the level of input. After reaching saturation, the transistor will then fully on / max power. then when the voltage drop bellow saturation, the transistor will work in linier / amplifier mode, and then finally turned off.
For explanation about different mode of operation:http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans2.html
So, the current flowing in the coil when the transistor is off is not the maximum current that the transistor can provide, but instead, it is the minimum possible. And worse, it is also possible that when the current finally stop, the induction current already passing the transition and will end up negating the coil collapse current.
If the goal is transistor swithing off the current after the transistor letting the maximum current flow, then the induction current from the trigger coil must be prevented from powering the transistor until it exceed the transistor saturation. So the trigger coil will either produce nothing or exceeding saturation, and nothing in between. Maybe this can be done with zener diode.
This is just my theory for why the SSG has very low efficiency.