Of course it is still early (or not yet even) in the "EOL" stage for all sets in this theme but you are correct that overall it has not been too good investment-wise.
By CAGR in the database, Shelob's Lair (25%) and Mirkwood Elf Army are the leaders thus far. Mirkwood Spiders has really taken off lately, Lake Town Chase got a huge bump over the holidays, and Unexpected Gathering is finally starting its inevitable climb as well. ToO speaks for itself.
The first four are very roughly double MSRP on eBay last I checked. Why those? The minifigs tell the main story, if you ask me - especially after the final Hobbit movie.
But to address your question, is any of this performance "good"? For the pair of wave 2 Hobbit sets, I would argue yes. They can still be found in the occasional Walmart for up to 50% off and are fetching ~4x that. But it really depends on when they plateau, of course.
On the subject of minifigs, Lonely Mountain has five exclusive version minifigs in addition to the big fire-breathing star of the show, including the three dwarves that "matter" in the movie besides Thorin. Perhaps more importantly, it represents the point of the whole dang story from start to finish. For purists, very little of that other "5 armies" spectacle was in the book, and none of the Dol Guldur business. Of course this probably won't matter a great deal but it is something to think about long term.
Helm's Deep is not a great comparison, in my opinion, though arguably the closest between the two sibling themes. It did do quite well initially after becoming unavailable, but plateau'd very quickly too. Holidays and all that...
It only has 3 exclusive minifigs out of 8 total, by the way, two of which are comparable to the Lonely Mountain dwarves in terms of being "second tier" important characters. No super-villain-huge-awesome-guy to carry the set at all. And unlike Hulk, we know we will never see another Lego Smaug.
Furthermore, despite a very solid design, Helm's Deep suffers from "lots of grey bricks" syndrome. The Lonely Mountain does not. It is colorful and therefore more unique.
Finally, Helm's Deep is just not iconic enough to represent LotR the way Lonely Mountain represents The Hobbit. It is much more similar to Lake Town in many ways, story-wise. The difference being, it survives.. mostly. Now, had Sauron "lived" in Mount Doom, and THAT set been released, WITH him as a maxi-fig, we would have an ideal comparison product.
I realize Unexpected Gathering is much easier to display, so it may be chosen "over" this set by many folks who don't give a rip about any of the rest of it. But I think the (personally predicted) difference in supply will more than make up for that in terms of the sets competing against each other.
Another way to put it is that this set is to The Hobbit what Black Pearl was to PotC, or Hogwarts to Harry Potter (imagine if there had only been one version). Obviously I am not the first to draw those comparisons.