First of all the guillotine is simpler, it doesn't require extensive training to use, you don't need to find a vein etc. Rocket sleds etc. are complex and prone to fail.
Who you think would deserve to be executed, is of course, based on what standards one holds.
I think someone like Cainkane1's rapist who crushes the throat of his three-old victim to keep her from testifying would qualify under my sociopathic, barbaric, standards. A tree shredder is a pleasant fantasy, but a guillotine would serve the same purpose. It also is simpler to use. Having the shredder fail partway thru would be a touch inhumane.