I played in States with a similar build and it was just horrible vs PyreZombie/Hammer and NetherGo. I'm still tuning however....
4 Cremates in the side are absolutely necessary to defeat those decks I mentioned above. However, you have to draw them, so you need a wee bit more acceleration IMO. 1 Soothsaying, 3 Fact or Fiction and 4 Accumulated Knowledge might be better.
Since Vendetta and Snuff Out are worthless vs Zombie and Spirit (unless you pack Distorting Lens), but good against everything else, I say side them too. 4 Massacre for white, and maybe a Rebel Informer. And maybe not.
I'm glad to see you haven't included the Recoil/Lobotomy combo, which is good, because even though I used those and they helped beat Waters/CounterWrath, they weren't that hot vs the other decks I mentioned.
You only have 6 creatures plus Undermines to do damage, which frightens me thanks to the currently strong presence of Parallax Wave. The Diamonds and Cameos are nice, but I have to free up space to add more creatures, relying moreso than you on acceleration and Rits.
Washout is wonderful but also a luxury. I'm of two minds about that card, and will reserve judgement for now.
Counterspells, Undermine, you bet. Prohibit is good, but not great. Perish I would never maindeck without Vamp Tutor.
I didn't play Dark Rituals, and now wish I had. They can bring out a Wumpus or Elemental early, which are two great cards, by the way.
Urborg Drake is terrible, just terrible. It's like a drunk that has to attack each turn, then it falls down when Nether Spirit attacks you. I know this from personal experience and wish I'd packed either Ribbon Snake or Drake Hatchling. Those two cards fly as well, and have a toughness>2, so you can Wump for two and they live. They can keep Nether Spirit at bay, and will draw out counterspells and Urza's Rage giving you a chance to have Wumpus or Elemental stay. I'm not a fan of either one of them, but they work, and they're ten times better than Drake, which is great vs most U/W and that's about it. I also had Unmask the Incredible Card but I have to take that out too since I'm mostly going Blue:
3 Air Elemental
4 Thrashing Wumpus
4 Drake Hatchling
4 Ribbon Snake
4 Undermine
4 Counterspell
4 Dark Ritual
4 Accumulated Knowledge
3 Fact or Fiction
1 Soothsaying
2 Lobotomy
Sideboard
4 Cremate
4 Massacre
4 Perish
3 Vendetta
4 Salt Marsh....so good in this slower environment
4 Underground River
10 Island
5 Swamp
That's just a start and that's my two cents. I really like designing for these colors, Erik, it's such a challenge.