The RGB interface bypasses the framebuffer memory on the panel, the LTDC peripheral transmits the entire contents of the framebuffer memory in the MCU in every refresh cycle. It still requires an LCD controller chip, but it can work with a panel that has no framebuffer memory (GRAM).
The FSMC interface writes data into the framebuffer memory of the panel. It will obviously not work with a TFT panel which has no framebuffer memory of its own. Smaller panels usually have one, larger ones do not.
Both have advantages and disadvantages.
RGB mode allows the MCU to directly address and manipulate the framebuffer contents, move blocks around with DMA2D, blending layers, preparing frames in advance (double and triple buffering) for smooth animation, provided the MCU has enough memory for multiple buffers.
FSMC requires no framebuffer memory allocated on the MCU, e.g. fonts and icons can be stored in flash and directly sent to the panel framebuffer. Useful for microcontrollers with little RAM. Display effects are slow and difficult. Framebuffer memory on TFT panels is not directly addressable.
