Devilbound Creature is easily the most uneven and confused templates in Pathfinder. Compare an Imp (CR+1) to a Pit Fiend (CR+1). Polymorph (self only) vs Quickened Fireball.
With that being said, how can we fix this problem? Racking/stacking spell-like abilities seems like the way to go, though it's difficult with the spell levels being so varied. Should it really be Natural Armor +4 and fire resist 30 regardless of Devil?
Can this be compared to Fey Creature, or (hopefully not) Half-Fiend?
And lastly, is there a way to make this PC-useable?
Templates as a whole are firmly in the 'GM-Permission Only' territory. Players cannot take a template unless the GM gives them permission. Templates can greatly augment characters well above and beyond the published CR increase.
For example, toss Fey Creature onto a simple Rogue and you can create a monster of a character. Give him some Agile Weapons and take the Invisibility and Teleport abilities and you've basically made yourself Nightcrawler.
Does anyone have ideas about how to better-standardize the template? Regardless of PC or NPC-usage, it is unbalanced. Anyone have ideas to bounce around?