Let me start off by saying: I can see why it would be irksome for players who have not had the time to invest in as many characters as is required by this game mode. That's a completely fair criticism. It can be frustrating to be essentially locked out of things simply due to different time management priorities than a game mode may allow for. However, I think there's a simple solution to this for future iterations that will immediately alleviate this pain point: Allow for a full cast of trial characters of each seasons featured elements. While still allowing players who have their own rosters built to use their characters to test their character and team building skills.
I also want to recognize that what this mode offers in terms of challenge, rewards, and unique gameplay mechanics are quite enjoyable. It's a real brain teaser to put together teams that will work well enough to tackle the challenges you're presented with, while maintaining enough 'useable' team comps with Vigor to complete future stages. It's really a great way to introduce team building theory to players who might not usually interact with that sort of thing. The random nature of the Generous Boons and Alternate Cast characters provided throughout the challenge also adds to replayability and the overall "puzzle" of team construction through each of the Acts. Not to mention, the content gold mine awaiting those who want to create trial character guides to tackle these stages effectively both in terms of completing challenges, but vigor use.
I think there are definitely some tweaks that could be made to make it more broadly accessible. But much like the Abyss' deeper floors, and namely 36* clearing it, I think it's nice that there is a level of challenge here that pushes players skills.
Another point in favor of IT's reward structure: it merely requires the completion of each round. As opposed to the Abyss locking certain reward tiers behind time gates or other completion criteria for each floor, players need only complete each act in order to obtain the maximum amount of rewards. A much welcome change.
Overall, I'd rate the game mode at an 8 or 8.5/10. I really enjoyed the team building "puzzle" for lack of a better word, finding out what works with the characters I've been given. The random nature of the Boons and Supporting Cast will make each playthrough different. And the rewards felt quite in line for something approaching 36* Abyss levels in difficulty.