OK, this is just a summary of my opinions. I can elaborate on any of them, but I wanted to keep this fairly brief. My comments are about wardens as such, not comparison to Artifacts.
*What is your first impression of the new Warden feature?
It is poorly designed and implemented, with many obvious flaws. Being sold as a replacement for a generally well regarded Artifact system didn't help either.
*What do you think are the pros of this change?
This is the part where I have to struggle to come up with something nice.
Ummm... It is possible that maxed out wardens would provide enough power boost to be useful and overcome the current drawbacks.
*What do you think are the cons?
The main ones are lack of clarity of how this work, lack of flexibility in team selection, Rock-paper-scissors being generally useless, too expensive in terms of merits
*What adjustments would you like to see in the future?
1) Remove "over 50%" requirement to activate warden.
2) Add merits as daily Forgotten Trial reward. Really, what happened to mention that you can farm replacement resources in that mode? You don't get merits, which is a very limiting factor.
3) Consider activating more than one warden. As in a team of 3 reds, 2 yellow and 1 green will get all 3 wardens. Obviously, red warden will trigger in fight more often, because he has more heroes charging him.