I didn't mind the 60 minute challenges, especially since you could do them both at the same time, but I do generally like endurance survival runs, it is my favorite game mode. But I understand if others thought it's too much, maybe you could make it a 60 min overall, so they could do 3x20 min or something, and we who enjoy long runs can do 60 min in 1 go. I would really not like defense missions, staying stationary in one place is boring and puts me to sleep, don't do long or any defense missions I BEG YOU! RANT TIME As a matter of fact remove Defense game modes, it goes directly against what warframe is, a fast paced action packed run and gun, hack and slash horde mode. Staying in the same place for extended periods of time is extremely boring, mobile defense just let's you go AFK for a bit if you're using limbo, it's not fun..
I do however have a problem with being forced to play with others if we don't want to, I'm personally a solo player but I manged to do the challenges with clanmates. I didn't quite like the Eidolon hunting challenge either, it also felt like it's forcing you to play with others. Yes I know you can technically do them solo but it's a big hassle to do so. Same goes with Vault runs, can be done solo but a bit or a nuisance.
The ayatan challenge was a bit weird, 5 in a week is very RNG if you have none in stock or they are all already filled up, I luckily had a bunch but had no stars (buff the drops, please! I can't get those ambers they are insanely rare) so I had to farm the stars instead which took way too long and felt more like an elite due to it's time consumption.
I also have a pretty big problem with 8 bounty missions challenge, it's not even an elite but it takes well over an hour to complete + very repetitive. Either make them an Elite or lower that number to 4-5, it's a bit too much as is.
This weeks Lua challenge was a bit of an annoyance because as a Solo player I had to replay missions over and over and over hoping I get the right challenge that can be done solo and hope I don't screw it up. Took me quite a bit of time to do, more than it should have and far less than a grouped run would be, but hey I guess that's a part of the Challenge I make for myself in choosing to play everything solo 😄
The rest are.. acceptable I guess since I don't remember them too clearly so they didn't make a big impact negative or positive and that makes them alright I suppose.