We've done both, and the Spyrers seem to fair pretty well in both situations. I'm pretty good at using terrain to my advantage, so it doesn't seem to matter if the table is 25% covered in terrain, or 75% covered.
The important thing to note is that the Spyrers are quite fast (with several of them being Fleet and/or having access to Scouts, Infiltrate or Jump Infantry), and are very capable of making maximum use of even minimum cover. (Small squad size of 1-4 models max makes this very easy.) When the enemy can draw line of sight to me, I can still usually claim a cover save, and I make sure that the ones that can't are those who don't need it (i.e. Orrus with their Power Fields and Jakara with their Mirror Shields).
I can see where hampering the other army's ability to maneuver and get line of sight at all might be a huge advantage of very dense terrain, but I've not played with terrain so dense that my opponent couldn't maneuver, so it hasn't come up.