I don't have any experience with the 204, but I guess I used a Swift for quite a few years, and it works.
It's always the same story, kill 'em dead, but don't damage a hide. This is subjective opinion, and none of it is 100% reliable. Seventeens will cause damage, hot 224s will cause damage, as well, depending on the bullet used. It is virtually impossible to tell a coyote shot with a 243 or a 22-250, and I have seen hundreds stacked in rows. Some show damage and some don't. You have spitzers and hollow points and heavy and light bullets and every kill is at a different range, with a slightly different presentation. And, it seems that I have tried all of them, but it's just a blur, looking back.
But, there is really nothing new under the sun. You have a bullet at a certain velocity, with a jacket of a certain thickness, and it has a diameter suitable for your application.
What you need to do to get consistant performance, is to limit your shooting to a distance that isn't too close and also not to far. A 55 grain bullet shot out of a 223 at a broadside coyote, at 100 yards, will probably exibit the same amount of damage, (or lack of) as a 220 swift at 225 yards; same presentation, same point of impact. Find a bullet that does what you want at a comfortable range, and exercise restraint, taking only those shots that are makeable, on standing broadside targets. If he is quartering away, or moving; forget it. You will see different results. Basic stuff.
The 204 has really caused some chatter on the Boards, and has the mystique, and some impressive down range numbers. But, is it in the cards? Will 32 grain bullets have the secret formula so that you never lose a runner, yet you never damage a hide?
I don't think I believe it. I have been using heavy bullets, for years. I think they smack harder, and flatten coyotes. The price I pay is a reliable exit. A managable exit, and a dead coyote that I don't have to track. I probably shot more coyotes with 52/53 grain bullets than anything else for fifteen years. Yeah, they kill reliably, but they also cause a lot of damage, but hunting contests, damage is less important than bang/flop. But, priorities can change.
As one writer said, the large caliber bullets steam through a coyote. I agree with that. I started with a 270 years ago but the problem I had at extended range was accurate bullets. Maybe they fixed that, but for me it is swimming against the current when 22, 24 and 25 caliber bullets are available and so effective. In other words, a good 64 grain Berger at 3800/3900 fps will steam through a coyote at most ranges, out to my personal 500 yard limit, at least. And, transfer enough energy so that the animal doesn't run off and get hard to find.
I use 62 and 65 grain bullets in various hot 22 cartridges and am well satisfied with the performance. I use (mostly) 85 grain in 24; and (mostly) 100 grain in 25. These are good hunting bullets with decent BC and they kill reliably. If I mess up a hide with a spine shot, it's my fault. Not the gun, the bullet or the caliber.
The 204 may have some application, and the shooter will learn what nitch it fills, in due time. Is it the Second Coming? Hardly. Is it a better "all around" choice than (say?) a seventeen Remington? That's what it looks like, to me.
Whether you neck an '06 to .277" or a 223 to 20 caliber, it involves no magic. One choice among many.
Good hunting. LB