The impact on safety from P2F are twofold;
1) from an airline's perspective: because airlines don't have to pay for training they will not be motivated to get the best pilot money can buy. Passive and docile (see point 2 below) 'button pushers' and 'bums on seats' will suffice.
2) from a pilot's perspective: by putting your head through a financial noose these pilots will be totally inhibited from addressing any safety issues. As a matter of fact a whole new generation of pilots is bred, who from day one have NO idea of any safety concept, but merely aim to please management and the person to their left.
Here's Harlan Ellison with an absolutely brilliant rant of how Hollywood 'pays' it's writers, scarily applicable to the airline industry as well: