The most intense pressure is self inflicted....based upon my experience flying EMS.
The Med Crews are generally Type A Personality kinds of folks....and far too many buy into the "We are Lifesavers who steal victims from the Grim Reaper...mentality." and even if unkowningly create an environment that can facilitate a Pilot making a bad call on weather.
Good weather....and plainly below minimums weather are easy calls.
It is the marginal weather that gets you in trouble.
Add in the fact that these spur of the moment SPIFR flights in the middle of the night do not allow for much time to prepare yourself.....as the "need" for a timely response is measured in mere minutes.