Forget termites.

Monstrous, mechanical, metal-munching moon mice are much better for ruining a wizard's day.  (Or a cool story seed; or both.)

Monsters in DFRPG follow their instincts.  They don't have instincts to hide their kills.  It literally never occurs to them that it could be a problem.  (Just as humans don't worry about all the stray hair, etc. that could be used for shapechanging, spells, etc.)

Now, warlocks can think of such things.  Except I doubt the normal inhabitants of the Never-Never really appreciate being used as a dumping site for bodies (or anything else).  Imagine the scene where a warlock is trying to convince the police that he has no idea about all those missing people, and a group of fae show up with body parts; "Excuse me, but I think these belong to you..."

Make a contract with the local ghoul population; it's easier.

The Fomori held the island, and fought against the Tuatha de Danann, defeating them once and losing (big time) on the re-match (The first and second battles of Mag Tuiredh; and don't ask me how that's pronounced...)  On the second fight, Lugh threw a sling bullet through Balor, knocking Balor's Eye (the original 'evil eye') out of Balor's head, killing him and pretty much deciding the war at that moment.

The Fomori were (mostly) hideous to behold giants, although one of them (blanking on the name) was fair to behold (and made king as a sop to the Tuatha after the first battle; sadly, he was good looking but not very bright).

There also needs to be a Baron von Strucker-type; an cunning, intelligent adversary who is also admirable in some ways.

I have a player who took a similar limitation; only real-world animals (which does include people).  I gave that a +1 rebate.  It's an excuse for the players to spend a LOT of time at the Zoo, (which is ANG in our game, so they'd be there anyway...)

The 'unstoppable' monster is a part of the general mythology that Dresden (and most fantasy) draws from.  At least. the monster who is unstoppable in terms of direct physical force; the hero generally has to find some weakness, or find another way to defeat the monster.  Which is represented by the concept of Physical Immunity and the Catch; find the Catch, and suddenly the monster folds like a cheap suitcase.  In the case of Ogres, a cold iron spear-head thrown with Air or Spirit propulsion will serious crimp their style.

I could see have immunity to Mental or Social stress in some cases; although it would probably be a one-off monster.

I think it's a question of options, rather than power.

A sufficiently powerful entity could probably over-awe and conquer a small nation with ease...but if they did so directly, they wouldn't survive human retaliation.  Given enough numbers and incentive, the default assumption in the Dresdenverse is humans win...against anything.  Oh yes, they would take a horrible beating and thousands of soldiers/civilians would die; but so would, say, Ferrovax.

Which is why the supernatural players work together to keep things secret.

Spell-casters are the sterotypical glass cannons in a direct conflict; 3-5 effects, and they're staggering, which means again, sufficient numbers can wear them down pretty quickly.  Thaumaturgy changes things, but takes time to prepare. 

Your basic caster, from the start, can throw battlefield-level damage evocations; the question is, how long can they keep doing that?  4-5 tanks, maybe.  And what about missiles and other objects moving too fast to be seen, and exploding too far away to be hexed?

Evocation and Thuamaturgy are force-multipliers, allowing for casters to apply force far beyond what similar refresh powers can accomplish...at least for short bursts.

So, it depends on the type of conflict.  In a direct, stand-up type of fight, direct powers (Inhuman/Superhuman/Mythic) levels of Strength/Speed/ Toughness count for a lot more.  In any contest where there's time to prepare, casters refresh is equivalent to 2x to 3x the equivalent refresh of any other powers...at least.

In last two examples, I think you're talking ludicrous levels of Refresh for a supernatural monster...or maybe 12-15 refresh for casters.

Now I have an image of Fae realtors sending spam mail to wizards..."Easy terms; low money down!"  "All loans approved; no one is turned away!"  "Be living in YOUR dream sanctum TODAY!"