How does an unchained synthesist determine his ability scores?

Question

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.

The unchained summoner that takes this homebrew archetype synthesist says

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.

Answer 1

I haven't personally researched it, but if your rules quote is accurate, then it's perfectly clear: you use the summoner's ability scores, only giving them a bonus if the eidolon has the Increased Ability Score evolution.

A barely-topical rant regarding the synthesist and DnD 3e wild shape:

Synthesist was never an amazing archetype to begin with, because the minor benefits of being able to melee yourself instead of telling your eidolon to do it were mitigated by the loss of action economy - your eidolon could melee just as well without you while you cast buffs and crowd control. The things that made wild shape so powerful, back when it used to replace your ability scores, didn't apply to the synthesist (the druid remained a tier 1 caster, had dozens or hundreds of choices always ready to pick what to transform into rather than one eidolon that can only be slightly modified at level up or by eating a prepared spell out of combat, wild armor, some of the best buffs in the game), so they weren't really as good as a normal summoner, they just had the option to look totally amazing in a way no other character could. The new one is even worse and can only look like specific existing creatures in the Bestiaries. Pass.