The original chained summoner that takes the archetype synthesist uses the eidolon's Strength, Dexterity, and Constitution scores instead of the synthesist's own, but the synthesist retains his own Intelligence, Wisdom, and Charisma scores.
While fused with his eidolon, the synthesist retains his own ability scores, modified by the eidolon's evolutions. A syntesist's eidolon cannot apply the ability increase evolution to mental ability scores (Intelligence, Wisdom, and Charisma).
Do both the chained and unchained synthesist use the eidolon's physical ability scores or does a chained synthesist use his eidolon's physical ability scores and an unchained sythesist use his own physical scores or is there another way to read that unchained sythesist archetype?
If the second option is correct, this seems like a really bad deal for the unchained sythesist, making the character extremely dependent on multiple ability scores: the unchained syhthesist needs Cha 16 to cast 6th-level spells, decent Int for skills, then Str, Dex, and Con for engaging in melee, ability scores that used to come from the eidolon. I get that having ability scores of 7, 7, 7, 14, 16, and 18, respectively, probably shouldn't make a viable character, but having a character so dependent on most of his ability scores being higher than normal doesn't sound quite right either.