This series is largely being criticised for "not being x-com" . And that criticism is correct in the same way that one might blame a bicycle for not being a jetplane.
Obviously the purpose of a bicycle is not to fly.
UFO: Aftermath actually continues the tradition and ideas set forth by the original X-Com developers in "Apocalypse" and in the unfinished project "Dreamland" which was canceled by Hasbro and the game's subsequent assets purchased by Altar Interactive who made the UFO series.
So in Aftermath they expand the ideas of the original developers in their last unfinished game meaning it has no relation to "UFO Defense/Enemy Unknown" or "Terror From the Deep" as both are turn based games, or jetplanes.
Theres many cool ideas to find in Aftermath, like a cover system and expanded training model, again like the one in Apocalypse.
Suffice it so say if you found Apocalypse to be a good game then you'll have a good time with Aftermath and the rest of the series by Altar. The second game "Aftershock" got some very nice mods and the third one is definitely the most refined one.
But they're all part of the UFO story which develops through the series and are all good games in their own way.