Sometimes the des/devs at wotc make great things. Other times they do this bogus shit...
I like this set a lot, but there were quite a few obvious mistakes here.
1. Why does Sultai get 3 watermark mythics? Why does that zombie delve card exist when we already have the flashback card from innistrad which was tailored to the same group of people who would want this? Why wasn't the mythic hydra given delve instead? Abzan fans should be furious they only got one mythic, regardless of how good the hydra mixes with the abzan. It's not like the clans are a full block theme, so there's not even a chance for them to get the mythic they deserved.
2. These shoehorned rares and mythics have me a bit annoyed. Why would they waste a rare for a clan on something like trap essence? Why couldn't that card be something a bit more proactive? Like a shock that draws you one card and puts a 1/1 counter on up to one thing? If I'm paying 3 different colored mana, I think that's fine. And that mythic serpent could be fine if it had been ANYTHING BUT A SERPENT. Why not just a bulky djinn or even an elemental? Those would have prowess. Oh wait, I know. It's a leviathan for cross-block synergy with that bounce spell that ignores leviathans! Good thinking on your feet, wotc!
3. Why in that cycle of enemy color watermarked creaturea did we need abzan and mardu to get mirror effects of each other? So what if one pumps power and the other pumps toughness? Did b/w warrior tribal need that boost for limited? Could have easily made Mardu's chief r/w. Remove trample and slap looting on the mantis to make it u/r. Would it have been that hard or bad? Hell, make the mantis a 2/4 and some random underpowered cubes might even want it. As for the uncommon spell cycle, can we just replace master the way with mortify and call it a day? Or did spells matter need more pushing?
That last one might not really be a big deal, but meh. I still think it would have been a bit better that way.