What is the most challenging concept about transistors? I think the classic valve analogy (AKA Art of Electronics "Transistor Man") works to a large extent. To fully understand the physics requires a lot more but in between, there are relatively simple equations that describe the main current flow as related to the control terminal's voltage or current depending on the transistor type.

To go from single transistor to multi-transistor circuits was a big leap for me, but most of it is understanding how particular subcircuits work and recognizing them as blocks of a larger circuit.