Dream incubation seems reliable enough as long as you really mean it, want it.
But while it may be reliable, it's not full-proof. This is because we tend to dream about things we are concerned with and that's not something easily controlled. No matter how much you focus on something before bed, it won't compensate for the true concerns you've experienced throughout the waking day. However, we do have a number of dreams a night so I imagine that kicks up the chances of incubated ideas finding their way into at least one dream. The point though, as I said, is to truly mean it when imagining in your mind what you wish to dream about.