Smith shouldnt really be underestimated on any angle yet he had really that advantage when it comes to reach but with some good timing with body shots then that can really be handled out easily.
Moving in weight division? I dont see much for this to be a problem.
To a great fighter like Alvarez you cannot put him at a disadvantage just check out his fight against Khan, Amir Khan has a superior when they fought but all he needed was one punch to knock him out, Smith has the height advantage but he needs to be strong to keep Alvarez at bay who will go inside and do bodywork on him.