Canny barely gets x4 Rain Missions, so by the time you've done the Level 6 SSD, it takes around two regular Raindrop Mission Alerts to upgrade a single Survivor.
One Legendary Lead Survivor takes around 50 to evolve from 3-Star to 4-Star, and Mythic Lead Survivors take 70. At time of posting, there are only two Raindrop Mission Alerts in Canny, both giving 25. At that current rate with my current Survivor setup, it would take 17 missions to evolve just my Lead Survivors. Not even the ones making up the Squads.
This isn't even taking into account Schems and Heroes. Just Survivors. They either need to increase the amount in the weekly shop, increase Canny Mission Alert quantities, or award Raindrops for more quests. This really, really puts a limiter on progression and introduces an element to progression that is required to grind to progress in any decent amount of time.
Source: https://www.reddit.com/r/FORTnITE/comments/u3bca4/anyone_find_that_pdor_is_an_unreasonably_rare/