You can run biogas engines on seed oil produced from trees, but not when you produce it from melons (i.e. it isn't beneficial in any way). Also melons are trivial to farm even without any mods. One of the biggest advantage of multifarms is that you can fully customize your trees via breeding, for example sapling tree needs a lot of saplings, should have many leaves, grow fast and be short (to reduce cost of cutting it down), I also add apples to the mix, wood tree also needs to grow fast, but should also be tall and have large base and fruit tree needs many leaves. This alone makes it more interesting then other ways of farming trees.