I don't knw, some of these are definitely great, some others not so much, and you lose on a lot of potential. You also lose a large amount of passives from all the minor nodes in other ascendency, while for Scion you get +40stat(lol) and +1skill point, which isn't very good. You also can only take the start of another class if you put 2points past the Ascendency, meaning you can't take another subclass, which tbh sounds like a complete garbage, since it's likely you'd be close to the starting area of the class you're mimicking in the first place(if you do a caster, you probably take witch/templar/sin ascendency, and also nodes in those areas for elemental damage/cast speed etc which would put you close to their start anyway). You'd be way better off getting one more subclass node than save like 5-10skillpoints, because it's unlikely you could make use of those skill points in a way that'd give you similar power as the subclass nodes.
Does sound pretty cool and will definitely see a lot of Scion minmaxing shit, but I don't know if it'll be OP compared to pure classes, at least I doubt it'll be for every type of build(probably for some though, you can get immunity to stun at full end charges or 10% on hit to get fortify which both sounds very strong defensively).