There can always be CRM issues, both pro and con, since there are multiple persons in the aircraft. In many, if not most programs, the med crew, even one of them, can veto any flight, or force the termination of the flight after it begins. However, IMO and IME, med crews tend to trust the pilot for the most part and don't often say much, especially with an experienced pilot.
Most pressure to take and continue a flight is self-imposed. The companies don't often push pilots to fly, and often it's the opposite. Helicopter pilots tend to be mission-oriented and self-confident, often over-confident. They take flights that should be refused, and continue them when conditions are deteriorating. It can be difficult to admit that one isn't up to the task of saving a life, even though it's not actually saving a life. Any EMS pilot who thinks (s)he's in the lifesaving business needs to be in another business, but there are lots of them out there. Once upon a time the hospitals and operators did push crews to fly regardless of the weather, but those days are long past. Now pilots push themselves. How to stop that, it is a puzzlement.