All of my experience (working at several area hospitals and then flying patients out from scene) is with Med Center air through CMC in Charlotte, NC. Every team they have ever sent (especially pediatric specialty teams or perfusion teams) seem to have been excellently trained, well equipped, ready to do anything, and have good communication and working relationships with their doctors and ours. When they show up our docs generally turn all care over to them, if they want to extubate the patient, change every drip, and do a surgical airway our doc steps out of the way and says "your patient, I trust you, take care of them and get them on out of here".
Those teams are well trained. It may take several years of ICU experience to get on those teams. Many of the team members are RNs and RTs who have an extensive scope of practice with high standards for maintaining their skills and must answer to their medical directors for all competencies.
In most states, RNs have a very expansive scope of practice. So do RTs. Few will ever utilize what they are actually allowed to do. But, at no time do they consider themselves doctors. Skills can be taught easily. The additional knowledge is what sets them apart. But, the members of these teams will have additional training but they are still not doctors.
The sending physician has also had a direct conversation with the rec'g physican and will usually maintain that contact until the team arrives. The transport team will also contact the rec'g physician and a member(s) of the ICU staff to give a full report before departing.
If EMS was to get central lines, would the oversight be as much? Look at the stats for intubation or even PIVs at some services.