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.
The exponential model tying collector current to base-emitter voltage is why I think most old-school references just treat BJTs as a fixed gain base-current amplifier. Of course it doesn't help that the gain is unknown and varies with process parameters!
The classic Voltage as Water in a pipe, or check valve comparison are destructive and misleading analogies that do not demonstrate the behavior except in isolation, and that narrow scope of isolation isn't provided leading towards purposeful struggle.
The ideas and practices of the gnostics in general are just stupid.
If you can't explain how a PN junction, or its composites actually function simply, you don't understand what you are talking about. As simply but not more simply than necessary, without using math.
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.