Regarding cars:
I think we must agree on that the more speed You carry when You go off the road, the more potential damage to the car and its inhabitants, no?
I think we also must agree that higher levels of grip, the more speed You carry when changing direction.
If we also agree that its the points of direction change that represents the highest risk of going of (turns.)
Then we can conclude with the fact that higher grip, equals higher speed in the risk areas, wich again equals more violent accidents.
There is a common misunderstanding in motorsport that to increese safety, You decreese engine power. This is the route that has been taken in rallying post group B.
This is not effective against getting speed down in corners, where speed is at its most dangerous.
To limit grip one have to regulate tires, suspension travel, and aerodynamic help.
Would limited grip cause the drivers to loose control? Off cource not. They would have better controll in the risk aereas beqause this would happen over a longer stretch of time (more time to react), and with lower speed.
Group B cars had their worst accidents on high grip surfaces, like Toivonen on the tarmac of Corsica, and during extremly high speed, like Vatanen in Argentina.
On gravel and snow their power vs grip factor was actually very good (safe), tough their construction was not (the placing of fuel tanks and so on.)
The modern day WRC car represents a higher risk, beqause of the higher speeds in the corners (even carrying the extra weight), but compensate some what by their much safer build.
Regarding mechanics:
I do not agree that the mechanics would suddenly forget to tighten bolts if they where part of the competition to an even higher extent. These are proffesional mechanics.