For effects related to type, a synthesist counts as both a humanoid and an outsider, whichever is worse.
Does that mean that a Synthesist cannot use spells like Enlarge Person on herself? It says that she counts as both, which would allow it, but then it specifies "whichever is worse," as if for any effect she either counts as one or the other and not both.
Ordinarily, a summoner is allowed to cast such spells on the Eidolon thanks to share spells; I can't imagine why the Synthesist would be MORE restrictive on that issue.
This would also affect other useful humanoid only spells, though I am having trouble thinking of many others.
"The synthesist also gains access to the eidolon’s special abilities and the eidolon’s evolutions. "
"Share Spells (Ex): The summoner may cast a spell with
a target of “you” on his eidolon (as a spell with a range
of touch) instead of on himself. A summoner may cast
spells on his eidolon even if the spells normally do not
affect creatures of the eidolon’s type (outsider). Spells
cast in this way must come from the summoner spell list."
my guess is the answer is no, this is likely to prevent lvl 8+ synthesists going huge.
I think Kierato has the right idea. I would guess that particular line is regarding hostile spells. i.e., if a spell does more damage to an outsider than to a humanoid, you take the extra damage as an outsider, etc.
"The synthesist also gains access to the eidolon’s special abilities and the eidolon’s evolutions. "
"Share Spells (Ex): The summoner may cast a spell with
a target of “you” on his eidolon (as a spell with a range
of touch) instead of on himself. A summoner may cast
spells on his eidolon even if the spells normally do not
affect creatures of the eidolon’s type (outsider). Spells
cast in this way must come from the summoner spell list."
I knew I should have specifically read the text of share spells! I figured it only said that you could cast any spells on the Eidolon that you could have cast on yourself, but apparently it also says you can cast summoner spells that affect any type. Thanks!
Also, it says you count as the worse one for effects related to type. Being a target of a spell isn't an effect of that spell. So you'd still be targetable by spells that only affect humanoids, and your eidolon (but not you) is targetable by spells that only affect outsiders, but if the effect of the spell is different for outsiders and humanoids (such as the Holy Word ring of spells), you take the worse one.