Don't get me wrong, I used to buy Karo by the gallon once upon a time. Then I started running into some pretty awful problems while working with the sticky stuff. Asides from the hordes of ants that it attracted whenever my clean-up job fell short of stellar, it also acted as a beckon for every flying insect within a three-mile radius when used in exterior woods scenes. (I even came down with a nasty case of insect bite poisoning after being covered with the stuff.)
It's a real pain to clean off of many surfaces, especially if it's been allowed to soak in for a while. Corn syrup blood is also notorious for staining fabric... be it clothing, upholstery, carpets, or even wallpaper. Not a great thing for film-makers who shoot on location (aka a residential dwelling) rather than building a set.
There's also the joy that actors go through when it comes to spending a lot of time wearing it, even moreso if the temperature is hot (as it often is in Florida) and they start to caramelize.
And let's also think about how much fun candy blood is to get out of long hair!
The only time I ever use corn syrup (or any food product) as a blood base nowadays is if it has to go in an actor's mouth. I call it "spitting blood".
When I started doing FX work outside of my own projects, it was pretty obvious that many directors were extremely nervous about having me spray blood all over the place even though their scripts called for it. Fortunately, I'd already gone through all the trials and errors with blood formulas in my own home and had developed a good sense of which type to use for various tasks.
It often surprises people to learn that it's not uncommon for me to use three or four different types of blood in one scene. I have various skin-safe formulas when the blood is being used on an actor, and detergent-based bloods that get spattered on walls and furniture. I use gelling formulas for "pools of blood" whenever possible because they are a snap to clean up.
How the blood is being used is also factored in to which formula is employed. When running a tube for dripping or gushing blood, I like using a thin and slippery base, like glycerin. If the blood has to "splat" (or literally stick to the wall), I have to use a thicker, heavier concoction.
One also has to consider how the blood behaves and how it will work for the effect. A big complaint I had back when I was using corn syrup was that the stuff beaded up and didn't look right after the initial application. Glycerin-based bloods can do the same thing. Throwing some gelatin into a blood mix works great for a thick, steady flow but once it dries the blood drip appears to defy gravity if the victim is mobile.
It pays to experiment. Whenever I encounter a new material, I always wonder how it would work as a blood base and then have a grand time testing it out.