I think when people refer to it feeling like "half a movie," they're refering to the pacing and character arcs and where they are at its conclusion. A cliffhanger isn't what makes something feel incomplete- if that were true, we wouldn't love Empire Strikes Back and Return of the King so much, and Dune Part Two would not be receiving the praise it's getting today. Even the previously mentioned M:I Dead Reckoning Part One ends on a MAJOR cliffhanger but still feels fleshed out and like a satisfying full movie. For me, the final 20 minutes of ATSV definitely cause it to drag, as well as the hard split between the half with the Spot and the half with the Spider-Society. By the end, the energy dies down for me and makes for a less-satisfying ending than many "part one" movies achieve (see the mentioned movies). I still love it, but the "half a movie" thing is a valid perspective.