Bike armies do suffer from terrain a lot. Terrain is my worst enemy: I can't turbo-boost through it, I can't go to higher levels inside it, I risk losing models when moving or assaulting into or through it. Since I suck at dangerous terrain tests (and terminator saves - I keep rolling those 1s all the time) this has proven devastating to me more than once. But forcing those situations is an act of desperation anyhow, it's not supposed to be necessary.
Bike armies have a huge advantage that makes up for the lack of "terrainability": speed. They are hardly predictable for the opponent and they can concentrate the firepower of their entire army wherever you need it, quickly redeploying to a completely different spot the following turn. This is devastating for the enemy and makes bike armies very competitive as long as you don't play them like a rhino-rush. If bike armies want to be successful they have to be played using a different approach. A little more cautious and tricky than the in-your-face Space Marine builds.
As far as objectives go, bike armies are the kings of the battlefield. Seize ground is my very favorite mission.
Bikes are your troops - you shouldn't contest with them unless you are very, very desperate. If you are, there are many tricks to accomplish your goal. What I did, for example, instead of running my bikes up a hill, was to move my lead bike within 5" of the outermost model of the enemy squad holding the objective (a model which was at the foot of the hill and therefore not in difficult terrain) and then assaulted. The counter-assault move of the enemy pulled the squad from the objective. I did not contest it, but the enemy did not hold it either. Objectives on the first level above the ground can be contested from ground level, as has been said before. Etc...
But your bikes are actually for holding objectives, and remember that you need only hold one (more than your opponent). You will always be able to at least place one objective, so putting it into the open is the thing to do. Your speeders and tanks are for contesting (and attempting tank shocks), so use them instead. The former can also reach the upper levels of ruins if need be, but if the enemy tries to castle up in there you can just sweep the ruins with mass firepower - something I have resorted to often enough. Apart from that bike armies work very well in combination with one mechanized tactical squad (which can easily claim objectives in terrain), and I have even often used a small squad of scout snipers with my bikes, who again have no difficulties with terrain. If you use many ruins you can also incorporate an assault squad into your biker army. It can't claim objectives, but very well assault the enemy unit claiming an objective and/or contest the latter.
Fear terrain with your bikes. But don't think terrain makes bike armies uncompetitive. Your bikes shouldn't be rushing into the midst of your opponent's lines and dashing through cover every turn. You should use them less aggressively, bring them to bear wherever you really need to (assaulting an Eldar pathfinder squad in terrain is risky but well worth it even if you lose one or two bikes in the process), and contest objectives with your speeders. They may die, but hopefully you have enough of them in your biker army...
