Grouping consoles from different manufacturers together when they don't follow similar release schedules is difficult. IMO consoles should belong to the generation they are competing with for the majority of their lifespan, otherwise you get into a state where Nintendo may release there 10th gen console just after sony/Ms release their 9th gen console and it makes no sense at all.
If for the majority of the NX's lifespan it is competing against the ps4/XBO it should be classified as belonging to that generation, so an 8th gen system. Neo is a revision of the ps4 in no way should it be classified as gen 8.5, if the trend of incremental hardware updates continues generations may be completely abolished anyway.







