While it may look like a good idea, for a speed limit to improve safety, they have to make drivers slow down on the dangerous bits. Drivers will only slow down if the speed limits are enforced and if the limits are lower than the speeds that the vehicles are already being driven at.
On many of the very narrow roads the safe speed depends on the visibility, so can be very low on tight corners, and can vary a lot. There could be a speed limit that is so low that all the bends can be taken at that speed, but it would be ignored by nearly everyone, just like double white lines are when overtaking cyclists. There could be speed limits that slow the conscientious drivers down on the faster bits, but to little advantage. There could be speed limits set for every bend, which would just add difficulty to driving legally, as well as being unenforceable.
I drive some roads well above the posted speed limit to avoid shouting matches with passengers and to avoid being overtaken by just about everyone. Here is an example:-
https://goo.gl/maps/GmZFB7Bb9P5VWPWz9 The image is old and the 5 mph limits are not shown. Here is another example:-
https://goo.gl/maps/nc1EXsoPJ2roVjz99The scheme sounds a lot like "Something Must Be Done", the 5 mph speed limits, or "Cyclists Dismount" signs.