The phrase underpowered is as much a subjective one as it is a factual one. I don't think that underpowered cars are dangerous, they just need to be driven according to their abilities. Different drivers have different perceptions of power and driveability as well.
Higher powered cars are easier to drive, that is without a doubt. Unless, that is, we're considering something designed for fast track usage, something heavily tuned which puts out 650bhp, yet hardly idles below 2000rpm, and won't pull at all until over 5000rpm then it gets very tiring on the road with the constant gearchanging (and the racket it makes!!)
I don't think that high powered cars are a problem if the driver is sensible with the application of the available torque and acceleration.
I also disagree with the 'bit of extra power gets you out of trouble' types of statements. It's the driver that got themselves into trouble in the first place.
With a shedload of extra torque and acceleration on tap it's very probable that this will get a novice (or experienced but bad anyway driver) into more trouble, twice as soon, at twice the speed (which means 4x the kinetic energy to shed or dissipate !!)
So, in short, neither is safer. Assuming proper maintenance and everything working (steering, brakes etc) it's drivers which make situations dangerous, not cars.
Here endeth the lesson