But, on the other hand, 20% isn't exactly a knock down, drag out fight. It's a couple of hit points and spells. I would hazard to say that a difficult fight should be into the 50% range or possibly better. I wouldn't think that after a hard fight, I blow off a couple of cure spells and keep going. For me, a hard fight means that I want to rest RIGHT NOW.
The problem with niche creatures is that unless the DM specifically creates situations to use them in, they aren't all that useful. I violently detest Aha Gotcha creatures. I always have. The cone of cold from the old OM was exactly that - an Aha Gotcha. It doesn't take any great tactical genius to have the OM turn invisible and get into position then blast away with a cone of cold. I mean, come on, this is pretty basic stuff. Even if I don't worry about the turning invisible stuff, using the cone of cold is a no brainer.
Only problem is, once I've use it, there's nothing else. The creature has absolutely no depth. It pops up, alpha strikes and then dies.
In other words, it's a trap. It doesn't have the hp or the AC to last more than a round or two against the party and, even if it does, it can't do any significant damage anyway. It's basically a mobile cone of cold trap.
This was the same problem with the Rust Monster. It's not a creature, it's a trap. It pops up, maybe whacks an item or two then dies.
Why bother making these into monsters? Trap stat blocks are five lines long. Strip out these one hit wonder creatures and put in stuff that's actually useful in the game and put them in with Green Slime in the DMG. If a creature has no real use beyond a single action, it's not a useful creature IMO. Creatures should be standing up for at least the five rounds of an average fight, not going down halfway into round two.
As I see it, unless you add in lots of mooks and whatnot to make the OM useful, the OM pops up, gets off its Cone of Cold, then dies. How is this a memorable encounter?
I shouldn't have to engineer entire scenarios around a given creature to make it useful. If I want to, that's fine, I can. But, a well designed creature shouldn't require the DM to completely engineer the encounter to its advantage in order to meet it's CR.