easy -0* loose all your feats, skills, and class abilities and become unable to gain any until you die and your brain explodes with a creeper's birth or are cured. I do love the nod to the animal world zombies that these yellow musks give.

On an interesting side note this template actually does not inhibit racial spellcasting, SLAs, and racial features that emulate class features; so it is an interesting template to toss on more exotic monsters like fey that often get spell casting or sneak attack as racial features.

Honestly I am on the fence whether yellow musk creeper deserves an asterisk, musk puff and consume intelligence are both capped at DC 18, which even at ecl 6 isn't that hard of a fort DC to hit and only becomes easier as levels rise and harder to optimize and leverage. Also if a DM chooses to remove the ability to make musk zombies altogether I think the Creeper would be -0 at that point if there was simply a cap on the number of zombies then it would probably be +0.

I suppose from the yellow musk creeper player point of view you want to target exotic creatures to zombafy, the best path for them might be to go cleric or druid even with the caster level hit and try to optimize a build that reduces enemies saves? Maybe hexblade would be a good choice here to get dark companion or something like that?

Could the Awaken spell be used on Yellow Musk Creepers to make them playable, or would we have to go with something like fiendish/celestial?

Hmm seems like going Fiendish Phrenic Yellow Musk Creeper might make this thing pretty decent.