The advantage of IAN are the similar SOPs between ILS/GLS/MLS approaches and NPAs. The only downside is that the APProach mode cannot fly non-straight approach trajectories (curved approaches).
The bigger difference is that in IAN one sets the go around altitude from the start on the MCP, whereas in LNAV/VNAV you have to select it up later. And of course, only one button to push, not both
