The other night, I had a big game with a bunch of my buddies. We usually try to get in a big one with everyone while I'm in town, but this one was a little bigger than usual. There was a little bit in excess of 17000 points per side, I believe. On the other side of the table were IG, Tau, and Blood Angels. On my side were Chaos Space Marines, Daemons, and Tyranids & DE (me).
We set up in pitched battle, 24 inches away, with a table on either side of the main one. Chaos, 'nids, and DE deploy on the front line, while BA deployed across from the 'nids, and Tau and IG deployed in depth throughout their own table.
We were soundly thrashed.
There were a few problems that I faced with Dark Eldar and Nids. Some of it was due to my list--my friends usually don't use flyers, and the Tau player (not someone I usually play with) brought 3. The BA player used a formation that turned 3 Stormravens into flyers. Tyranids have no good defense against flyers using the current Apoc rules, which screwed us over to some degree. But I could've proxied Tyrannofexes, so I can't really complain.
Something more important, of note for all Apocalypse games, is however, board size. Large boards (more than 2x normal size, near as I can tell) take away large advantages for both. First, the Tyranid.
I simply ran everything I had, about 2000 points of normal tyranids plus a Scythed Hierodule. My force was concentrated into one side of the board, across from a blood angels column and some miscellaneous guardsmen. They were deployed back, towards the edge of the main table. And I found that I really struggled to reach them. Tyranid lack the mobility to advance on a board of Apocalypse scale.
Next, the Dark Eldar. Our main advantage is our mobility. When we can cross the board in a turn, we do well. When that merely brings us into the teeth of the enemy guns, we do less favorably.
And that brings us to the crux of the matter. USE TERRAIN! Lots of TERRAIN!!! We skimped on it because it would clog up deployment, and daemons would have trouble deep striking. Tyranids and Dark Eldar need terrain when facing an Apocalyptic gunline from Tau or IG. There is no way to make it across a table and a half with paper planes or footslogging monsters without cover and such to utilize.
My two cents, not well edited or thought through. A bit of tl;dr word vomit.