Biodiesel is biodiesel ,and SVO is Straight Vegetable Oil.Two different times of refining.The problems you talk about such as gelling are with the Svo.I think the problems right now are lubricity and a lower btu factor as compared to straight #2 diesel.
Bio well gell too, it depends greatly on the feed stock (oil source) used and the way it is processed and dried. I suspect that is part of the reason commercial bio is kept at a lower ratio, to prevent 'overwhelming' a dino diesel powered vehicle.
Bio tends to loosen a lot of dirt/sediment etc when first introduced to a fuel system, therefore plugging/requiring a lot of fuel filter changes the first while. To test a fuel for cold weather properties, pour some in a clear jar, put it in your freezer. Watch it and see what it does as it gets colder, if there's is a lot of water in it, ice crystals may even form, otherwise it will slowly gell depending on it's cold weather properties.
WVO requires it to be heated prior to injecting, to help it atomize small enough to burn properly. Again, temperature depends some on the quality of the original oil. Most guys going this route, particularly in northern climates run a second tank, (often heated) and then heat the line and filter assembly as well. Sometimes even the injector lines are heated. A setup of some sort to switch from dino to WVO (and back) is also there. Start up, and shut down are performed with regular diesel.
Economy is gained by anyone willing to either collect and use WVO, or process their own bio from the WVO, however, it can either be time consuming, expensive, or both. If you have the time however, it can be done fairly reasonably if you research, setup, and get things going in a way that works for you. One additional benefit is the lowered demands on the petroleum resources, and some also claim that the emissions, since they were offset by the plants grown recently to produce it, is somewhat offset, as compared to dino diesel.
To me, growing plants to produce it and no other use doesn't make a lot of sense, however, recycling oil that was needed for other uses and is now being 'discarded' can make sense if done right. In some areas, that used oil is simply sent for landfill.