I like going through the Variant Dungeons casually, my biggest issue with though is the first boss. Why is it the same on every route, just with slightly different mechanics?

As for Criterion, I haven't cleared any of them so I cannot comment on the fight designs other than Silkie, but I find the most common complaint is simply lack of rewards for doing something so difficult. They don't have to create new armour sets, just make it so the first time you kill a boss each week you're given a token, with the final boss dropping 2, and then you can exchange 4 tokens to upgrade tomestone or normal raid gear. Since these dungeons come out on odd patches, it would be an alternate way to gear up other jobs before the next savage raids are released.