The skill's description says it "renders the target immune to debuffs".
Does it mean it is supposed to give absolute immunity to all kinds of debuffs and abnormal conditions, or there exist certain exceptions?
e.g., can it protect from rogues' Shadow Chase?
about the bolded part, surely yes for everything that can be put under the debuff line. For attacks attained with debuffs, like jump attack/stun impact/eruption etc, you'll receive the dmg, but not the debuff.
There's no exception regarding to pvp, but only with some sort of raids debuffs (i.e.: even with AT on, you'll be stunned by Ornafi inside Fortuna).
Actually no. If you resist the debuff, you take no damage. There might be some minor exceptions since I haven't extensively tested everything. Eruption and power bomber deal exactly zero damage if the debuff is resisted.