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It seems a rather bad tactic to me because you can't assault after disembarking. I would much rather put nasty shooting units in a WS like Fire Dragons or DA squads (with BS).
Any thoughts, am I missing something here?
I think the reason that' CC units would be put into transports is to save them from being shot if they were just running across the board. Banshees for example have a 4+ save, and move at max 12" a turn. But in a transport they can move 24" (36" with star engines) and assault the next turn and be safe from things with an AP of 4 of less.
It provides a faster and sometimes safer way to get the unit where it needs to be and where it can do the most damage effectively.
As soon as they get out (or hobble out of the burning wreck) they will be shot to pieces by rapid fire weapons or assaulted. Granted if you have Acrobatic banshees that's not that much of an issue but I still prefer to hoof forward to a nice piece of LOS-obscuring cover and wait for the enemy to come within charge range.
the thing is, that a wave serpent do block LOS, which can be used in a good matter.
i would put som CC units in a WS for the speed, survivability and in scorpions i like outflank, and outflank by vehicle is nice.
there's two trisck with eldar vehicle assaults:
1) you get out the following turn.
here's the situation:
You fly your Wave serpent 24" to within 6-12" of your enemy. You have your fun eldar vehicle protection toys and a 4+ cover save from having moved so far. Assuming you survive the shots you'll take during your enemies' turn, on your next turn, you pop out, move your normal 6", fleet if necessary and then assault.
if the vehicle gets hit, you still can do the same idea assuming you pass your pinning test (which is why exarchs are important in this case) though now you have a difficult terrain and possibly a dangerous terrain test to take.
2) you get out this turn, but you use the vehicle to provide cover and /or block line of site to the troops that just got out.
Because your enemy must stay at least 1" away from enemy models (including vehicles) and because they block line of site, geting out of the back of a waveserpent can be a fairly safe excercise if you plan it right.
Your enemy has two choices, he can get closer to you or he can run away. If he gets closer, you've now made him move according to your wishes, so you've disrupted his game plan. He shouldn't be able to assault, and if you choose your spot properly you should at least get some cover.
If he runs away and is now completely out of assault range (12" + d6) , pile back in at the beginning of next turn and do it the again the following turn. It's a good way of herding the enemy to where you want them to be.
it's not as easy as using a landraider or an open topped ork / dark eldar vehicle, but it can be very effective.
Now that you can load any unit into a waveserpent, you can, for example, take a WS with the dire avengers but load your banshees if you feel the need.
But seriously, this guy has it right. Wave Serpents with CC units can be quite powerful when used properly. Step one is having them disembark only on the turn they will assault so they are safe within that AV 12, shielded, 4+ save shell. If you park it close to an enemy they have the option of moving and being less helpful to the rest of their army, or sitting still and being dead, sounds like a win-win to me. With only a 6" move to get away your 2" disembark + 6" move + d6 Fleet + 6 inch assault is more than enough to catch them should they chose to flee.
This tactic was very popular last edition, and is still popular today. Often people fielded entire armies in wave serpents with as many banshees and dire avengers and possible. It was quite devastating (with that and falcons you can see why skimmers got nerfed) with almost no counter when used properly. Of course, in my opinion an army like that gets rather old after a while when you have such little variety.