There are probably few cartridges below .30 cal that can be expected to dump every coyote on the spot absolutely 100 percent of the time. (And probably a few .30 cal too). They can be remarkably resilient animals, and if you do enough searching and asking around, you will find reliable accounts of coyotes being shot with 'big game' cartridges that ran 200 yd or more with their liver hanging out. (More than a few three legged coyotes out there too).
I guess the 'Coles notes' version of what I am saying is that there are a number of factors that produce reliable kills, only one of which is the cartridge.
If you still have doubt about the ability of a .20 cal to reliably kill coyotes, I suggest you check out the Predator Hunt Canada website and look to see who won (not me) the little contest that was run there this year (and each year).