Mutons are literally just. Mutons. They're in the genebanks. There is nothing particularly 'new' or 'hard' about making them. They're big, bulky, brawny, and have armor. That's all there is to them.
And this is an arms race. Which means anything we design can be determined with a dice roll, so, considering that Mutons are a mid game threat, and we are only on the second month, I would suspect them to be harder to make then the Vipers.
Vipers: Have functionally no armor. Scales are not armor when they're about as resilient as the average gecko.
Lasso: Because pulling a soldier to you when the soldier has a combat knife and a gun is totally a good idea outside of video games.
Poison is good but thinmen do it better.
Thinmen use carbines. Not rifles. Again thinmen are superior to vipers in every way that matters.
From the Xcom 2 wiki "Despite their resemblance to terrestrial snakes, the Viper is actually a wholly extraterrestrial species unmodified by human DNA. However, it retains the modified Thin Man's poison and high accuracy, combat abilities and intelligence." And, from the multiplayer, they have poison spit just like the thinmen had, so, I don't think thinmen do it better. It also never mentions just how tough their scales are, but a knife probably would be useful if you could manage to pull it out quick enough and use it after being pulled violently and quickly from where you where standing before having the equivalent of what a boa constrictor would do with a much larger snake done to you.
Chryssalids - 11 terrors out of 10
In Terror of the Deep there were flying octopuses who could do same trick, just they flew.
Edit: And what of Chimeras? They seem to basically be Mutons with basic healing factor.
They didn't fly actually, they only where on missions where you where under water so they could only swim.