I guess I don't see it as such a problem that rogues nor clerics will be especially effective against this baddie:
-Rogues are always nigh-useless against undead.
-The cleric has lots of other cool toys besides turning undead that can work against this (magic circle vs. evil, gate, etc.)
Although I've not played a game at higher levels, I've watched games like Piratecat's, and seen how incredibly difficult it is to challenge high-level PCs. A lovingly-created unique enemy with all sorts of nasty powers can easily find itself taken out in a single round by 17th-level PCs.
The way I see it, the effigy has decent defenses against some of the more basic dweep that you get at high levels: no sneak-attacks, no turning, no crits. That doesn't mean it's unkillable: it just means that you have to use other tools in your kit.
-Use one of several spells to fight it on the ethereal plane (I think this'll work)
-Pull out all the force-based spells you can think of
-When it's possessed someone, open a gate to the plane of positive energy and bull-rush the possessed victim through the gate.
-Use spells like holy aura on its possessed victims.
These are all things that a cleric can do. As usual, the rogue should put that Hide skill to good use until the nasty ghostthing goes away.
Because of the huge number of save-or-die tricks at the disposal of high-level characters, I think it's appropriate to have the occasional critter that negates some of the options.
This creature is vulnerable to any spellcaster and to any fighter with a magical weapon (although it's extremely nasty against an unprotected fighter, whose low Will save will doom her to burning from the inside out). The fact that it's not vulnerable to three particular tricks, two of which are invulnerabilities shared by your average zombie, doesn't make it poor design, IMHO.
However, I agree that "Effigy" is a fairly stupid name. Get it? "Burned in effigy"? A-hurk. A-hurk. A-hurk.
Daniel