How dangerous are these cards?

Over at Goblin Artisans I've got a series of "Here's what I learned" posts focused on the game design of Guilds of Ravnica (a set I really, really like).

In the entry on Izzet, I delve into how much work goes into creating a "spells matter" thematic identity that is draftable and competitive in limited environments. The lesson here is that it's a LOT of work and it's hard to do in sets that are designed around 10 color pairings being draftable. 

So, working through that is making me think about the challenges of trying to make spells matter in one way in U/B but scrying mattering in R/U and then reave in R/B and whether it's possible to design it all. I'm thinking of dumping the scrying matters concept (I'm actually thinking of dropping attempts to design for enemy color pairings entirely). 

I have thought about bringing flashback in to make U/B work better. But if I do that I'm giving the two colors way too many mechanics compared to the others. I could take mutate away from black. I might do it anyway. The color just doesn't seem to want to play with it and flavorwise, fecund is actually a better tool to show how disease and plague are being spread.

But rather than flashback, I thought about more ways to interact with the reave kicker to make and black and blue identity without bringing reave into blue.

Are these cards too powerful in the limited environment of Overgrowth?

These would be utter trash outside the game environment of Overgrowth. But it feels like they're really powerful within Overgrowth itself. Maybe they need to be uncommon? What I won't really know until playtesting is how frequently a player will be exiling a card from the graveyard.