I'm struggling to follow the logic of the small units argument for either Howling Banshees or Striking Scorpions, but especially relating to the former. In the case of Howling Banshees, it only takes a few casualties to effectively neutralise the combat effectiveness of a squad of five, so that even if any survivors successfully engage an opposing squad they are still likely to lose by weight of numbers. Striking Scorpions are more durable, so are less susceptible to this problem, but it can still happen.
Yes, it's much easier to neutralise a smaller squad, however for the same points as a larger squad you can have 2 squads, since the FOC does not really have a cap on how many units you can have this won't be a problem. Also Exarches are now very powerful and well worth their points, small units means you can have more exarches. A unit of 2 banshees + wounded exarch, can still take out a combat squad of marines.
While it's true scorpions survive longer than banshees (even when considering ppm) in general, but when they manage to cause fear, they are just as durable as the scorpions, due to not getting hit as often.
I found that units of five or six Howling Banshees and Striking Scorpions were too fragile once assaults from moving vehicles were disallowed, and this became even more of a problem once the rules changed to prevent assaulting out of closed top vehicles period. Eldar assault Aspects being fielded on foot need numbers on their side, so that, in conjunction with cover, enough of them survive to remain a functional force when engaging opposing units in an assault. To this end, I remain of the view that squads of ten are the best option.
Assault units do not need a transport these days, with battlefocus eldar infantry is almost as fast as the vehicles, unless going flat out with the vehicle, and with matchless agility, only the 1-2" you can earn by spinning the vehicle differs.
If you expect your units to take attrition and try to make up for it by taking larger units you are not playing towards the eldars strengths.
As you used to say, eldar can't win a war of attrition, which is still true.
That's why I find it more useful to play proactively than reactively. If you play reactively you are also more prone to fall into psychological traps.
Deathstars aside most armies don't use units with more than 10 wounds in them, that's why I find it more useful with smaller units, also I would try to avoid pitting eldar CC specialists against opponents CC-specialists, because eldar simply won't do good in fair match-ups. Here is where I tend to take advantage of the eldar speed, by choosing my fights.
If I have two units, one shooty and one CC, and my opponent has the same, I'd much rather have my CC unit going up against the shooty opposing unit, and shoot their CC-unit, than throwing my CC-specialists into a brawl and let the shooty units have a firefight of attrition.
@Banshees vs Scorpions: If I had to choose only one unit of these I would take the scorpions every day, but when I'm allowed 2 units, I always take banshees over a second unit of scorpions.
The comparison 10 banshees with exarch and executioner comparred to 10 scorpions with exarch and claw, is not even remotely fair. You could actually buy the banshees a DE-raider for the points in between, or almost get 5 additional banshees.
Here's a more fair comparison:(5x banshees(including Exarch+Executioner) vs MEQ = 4 kills.)
x5 = 20 kills /425p (33 plain banshees will only achieve 17,6 kills)(10x Scorpions (including Exarch+Claw) vs MEQ = 8.33 kills)
x2 = 16,67 kills /420p (This includes the bonus attacks from stalker and the mandiblasters)Survivability against bolters
Banshees = 90 hits
(33 plain banshees: 99 hits)Scorpions = 99 hits
(25 plain scorpions: 112,5 hits)This does not consider cover or overkills.
As for getting into CC, scorpion can usually infiltrate 6" closer, however they may not assault turn 1, so for turn 2 assaults, the effective range of scorpions and banshees are:
Banshees: 6+3+D6+6+3+2D6 = 30"
Scorpions: 6+6+D6+6+2D6 = 30"
(Counting the re-rollable dices as 4")Multiplying the killyness and survivability together we get:
Banshees: 20*90= 1800
Scorpions: 16,67*99= 1650
1800/1650= 1,09
420/425 * 1.09 =1,078
That makes banshees 8% more powerful vs MEQ