Another thing that makes peds tough, and something that I think a lot of people over look, is the difference in patients of different ages.
In IM, treating a disease in 25 y/o, 40 y/o, 65 y/o, and 90 y/o is pretty much the same.
In peds, a 24 week premie, 35 week premie, normal neonate, 2 y/o, 8 y/o and 15 y/o are all very different. They all have unique anatomy and physiology. They have unique nutritional requirements and are at different stages in neurological development.
In regards to cardiology, adult cardiology is fairly easy in the sense that the big diseases that they treat (MI and CHF) are so common that there are so many good studies, each with a clever name (like ALLHAT or COPERNICUS), each enrolling like a billion patients, that they all prompt the establishment of protocol to do for a patient of a certain demographic. And you see these diseases so often that it just becomes habit...ok MI...ok give them O2, nitrates, beat blocker, ASA...check enzymes, CXR, EKG. Done. I think something like 6 million people are diagnosed with CHF each year in the US. Only 8/1000 kids born every year will have a heart defect. In peds cardiology, specifically congenital heart disease, each child has a unique defect. With the exception of tetraology of fallot, rarely do children come in with a similar defect(s) as the kid before them. Even with T of F, the spectrum of disease varies a ton between kids. I have heard of cases of T of F not diagnosed until the patient was in their 30's. With the other defects that you learn about (such as septal defects, valve abnormalities, and major artery/vein abnormalities), they often have a unique combination of defects. Some of the defects are so rare and each kid is so unique that it is impossible to have good clinical studies that guide treatment protocol (assuming it awas ethical to do research on kids). Each kid requires a creative mind of the surgeon and cardiologist to determine what the best course of action would be. This is what makes the field challenging and incredibly fun.