Under FAR Part 135 require an Operator to maintain operational control over all aircraft and flights. In the past that was ignored by both the FAA and Operators less Operations Manuals and 135 Ops Specs setting forth the weather minima and related information by which the Pilot-In-Command was to conduct the flight.
After several years of high loss rates of aircraft and personnel the FAA was criticized by the NTSB and industry critics for failing to ensure Part 135 Dispatch requirements were being complied with. That brought about the creation of centralized dispatch centers for the various EMS operators.
Since that occurred, the EMS accident rate has improved greatly.
Industry practice has been to consider the EMS flight as Part 135 (Air Taxi rules if you will) but revert to the more liberal Part 91 (Non-commerical rules) for the flight home after dropping off the patient(s). The weather minima for Part 91 regarding night flight and weather minima are much less restrictive than under Part 135.
Picking a stat off the top of my head....if my memory serves me right....80% of fatal accidents occur "after" dropping off the patient and as the aircraft is on its way home or to a refuelling point prior to the flight home.
CFIT accidents resulting from indavertent IMC while operating VFR continue to plaque the industry for a number of reasons. We have a lot of "dark" areas with few or no weather reporting thus unexpected weather can be a direct result of that. Throw in uneven terrain and no NVG's....and the trap is set.
Despite some operator's propaganda...."IFR" programs sometimes really are not as the budget does not provide adequate training or opportunity to stay "current and proficient" although by means of a checkride to minimum standard every six months makes one legally "current".
There is a world of difference between doing a practice approach on the way to the fuel point without screens or foggles and encountering IIMC at night while being only legally current.
Planned SPIFR beats heck out of scud running but as an industry that is what happens more often than not. Being fully equipped....including training and real currency is the cornerstone of SPIFR but that is the hard part to maintain in a working EMS operation.
The price we pay for falling short of the mark is dear.