More games under my belt today, one vs. infantry Space Wolves and one vs. Knights - so pretty much both ends of the spectrum.
It strikes me that the core Eldar issue is that the Eldar, not having had a range update or new units to speak of save the fliers since 4th Edition - simply aren't suited to the superheavy era. They've always been excellent against infantry and good at targeted, mostly short-ranged strikes against vehicles - but even things the size of Land Raiders could give them trouble back in the day (in principle. In practice Land Raiders tended at least when I was last playing to be drastically overpriced for their firepower, as they paid so much for durability). In a full game my army succeeded in destroying one of three knights and coming close to taking down the second, but there was never any prospect of getting them all - and that's with what dedicated AT we have access to (Reapers, wraithcannon, Hemlock and Fire Prism). Nothing the Eldar have save the Scorpion seems remotely tailored to taking out superheavies, and the entire 'fast but fragile' motif goes out of the window in a world where things with the firepower of superheavies exist that penalise fragility so heavily.
Some of these 'problematic' units seem to work fine in more traditional games.
Fire Prism: We have this flagged as perhaps the least efficient unit in the list. I used it, reluctantly, because it really does have a role no other non-FW unit in the army effectively fills (save the Night Spinner, which I don't have). It overperformed today - my rolls were especially good with it, but it confirmed for me that this unit is good enough in its role if something can be done to mitigate the random element.
Fixed damage on the weapon modes (2-3 for focused, 6 for lance) and an ability allowing it to reroll the number of shots in focused or dispersed mode (even just rerolling 1 for the number of shots might be sufficient) is enough to make the unit fit for purpose. Even if that results in slightly lowered damage output relative to dedicated AT units like the Predator, the Fire Prism has a better unit profile (faster and with more wounds) and better anti-infantry capabilities.
Its cost is likely to still need tweaking - it's still inefficient relative to the Night Spinner - and there ought to be some rebalancing of the modes to make lance more attractive (a reroll ability already helps dispersed somewhat), but I don't think it's in as dire a situation as it's been portrayed.
Dire Avengers and Windriders: Nothing to say here. These units repeatedly underperformed and even when fairly effective don't justify their cost. Possibly I've been using them badly or haven't had good targets, but at this point I'm temped to bump shuricannon Windriders up to 'significantly overpriced'.
Farseer Skyrunner: I don't recognise the 'significantly overpriced' flag for this unit. The Farseer is an excellent HQ choice and the mobility of a bike has proved to be worth much more than I expected. It looks a very expensive upgrade, but the ability to smite more or less wherever he wants (or Doom or Guide, but since those aren't restricted to the closest unit that's less often relevant) and to almost invariably avoid being targeted at range is hard to put a price tag on - he benefits much more from being on a bike than combat characters like the Autarch.
Eldar Missile Launcher: Continued to perform well for me, but as its damage is variable and blast weapons are weaker than they once were I'm on the cost reduction train. I don't think it needs stat changes, and if it cost the same as the bright lance does now I'd most likely want the EML most of the time, so it probably either wants to be about 3pts cheaper or 25pts and the bright lance slightly cheaper or with a damage buff (2D3 or perhaps 3+D3 rather than D6).
Hemlock Wraithfighter: This unit should be more expensive. Not by much, perhaps (20-30 seems good), but it's sufficiently hard for non-specialised armies to harm even with its short range that I never even needed to use Conceal, and for a unit that can't be answered its effect on the battlefield is too good.
If we're setting as our baseline for appropriate costs the 'good' units in the table SeekingOne put together, there's little question that the Hemlock is substantially better for its cost than the other units here. It's already been pointed out by others that it's both more durable and with better average damage output against most targets than the Crimson Hunter - and that includes the Crimson Hunter's optimal targets. Yet the Hunter is only around 30pts cheaper.