Well that was a hell of a lot of fun!
I think the difficulties were about right. Easy, challenge and brutal sums them up perfectly.
Challenge was good fun and I had my ass handed to me several times in brutal before I started winning.
Thanks to the PP team 
Fun? For me it was more brutal than Brutal difficulty. 
Well but now when I think about it… it was also more fun than Brutal.
I get wrecked in Brutal, but win Challenging at ease, so something is off about my tactics. Probably because, as I said, putting the heavy on a rooftop is a valid tactic to surpass Challenging, since the most the Chiron will do is goo him, and the other aliens will hardly get a hit.
But I’ll agree as well that these scenarios are more fun than regular Backer Build 
Maybe this is saying something that tactics are, at this point, more fun than strategics?
For sure. Without base building, without diplomacy, without proper manufacturing and research geoscape is not funny at all. Maybe except of some funny events. 
Even single store for all the bases feature is kind of broken. How the hell you can reach from Manticore more than few thousands kilometres away to you base storage and change equipment for any onboard soldier? 
And aliens are easy to exterminate when you soldiers start to have level 3 and above., so clearing the nest is piece of cake and in this way you assure that none of stronger aliens will appear (at least I think so).
I think the regular backer build has the chance to be as good as these challenges but is just a little broken at the moment. The second the Queen, Chirons and Sirens start turning up in missions, EVERY mission is like these challenges… we just cannot enter them at the moment.
I am really looking forwards to seeing the diplomacy and base building.