I enjoy the challenges overall but they can be very unfriendly to low and medium powered rosters because it is almost impossible to complete many of the feats on the lower tiers.
For example, today's challenge requires: 1) completion; 2) attempt to apply Thermal Detonators 40 times; 3) Score 3 critical hits in a row during ally turns; and 4) Full Jawa team.
There's a minimum tier at which you can do the Thermal Detonator feat because enemies will die too quickly otherwise. If it's tier 7, that means players who can do that tier not only get 17 more points than players who can do tier 6, they get 35 more points.
I would suggest having these types of feats scale. For example: 5 Thermals at T1, 10 at T2, etc. until you hit 40 at t8+.
Same with the Jawa requirement. Very few players have geared full Jawa teams. My guess is that people are either going to get 40 points or 0 points from this feat. Why not have it start at 1-2 Jawas and ramp up over the tiers?
Although I used specific examples, these sorts of issues persist since every GC has a team requirement and some challenge that's impractical to do at low tiers (get a buff 20+ times, etc.). I think scaling them better to be less "all or nothing" would be a huge improvement to this game mode.