Both Sea Gate Wreckage and Conduit of Ruin + Ulamog proved to be a god send vs. Miracles. I think it's a mistake not to play them. Conduit has been so solid that I'm considering cutting a Revoker to play the 4th Conduit.
Sea Gate Wreckage is a far better way to combat counterspells than Cavern of Souls imo. The main cards that you don't want getting countered, Chalices and to a lesser extent Warping Wails and Revokers aren't protected by Cavern. In fact, Cavern makes it such that those key cards almost always end up being the ones that get countered. Wreckage on the other hand blows the game wide open and outright wins games all by itself if your opponents manages to stabilize while taking you to top deck mode.
Eye of Ugin is too explosive when paired with Mimics and Endless Ones not to play as a 4 of IMO. The occasional hands you have to mulligans are more than made up for by the hands that have you dropping down anywhere from 2-5 2/2s and 2/1s on your very first turn. Likewise, the acceleration that City of Traitors and Crystal Vein provides are well worth having to sac them in order to let you empty out your hand a full turn or two earlier.
Urborg effectively lets Ugin generate 3 mana, while also letting you use Ancient Tombs to make mana without costing you life.
I've tested this deck quite a bit online and while there are certain times that paying 2 mana for TKS and other Eldrazi seems very broken, at the end of the day you are still essentially fighting "Fair" with creature bodies on the ground. One of things that makes MUD a potent deck is its ability to just hit the "I win" button either with an unbeatable answer (forgemaster into Platinum Emperion, or Sundering Titan) or its ability to generate insane mana with Metalworker early on into some bomb. Eye of Ugin and Eldrazi Temples make the Eldrazi hyper-efficient but none of them are really unbeatable answers except for maybe Ulamog, and this deck is more about the midrange Eldrazi. Midrange Eldrazi are much better in Modern where the power level is more even, but in Legacy it still doesn't feel like it is enough compared to Forgemaster or Ugin shenanigans.
You're correct, that's the flaw with the approach outlined in the OP. There is an alternative however...
Playing a manabase similar to the below allows the deck to support both explosiveness and a long game with game ending threats...
4x Eldrazi Temple
4x Ancient Tomb
4x Cloudpost
2x Vesuva
3x Glimmerpost
3x City of Traitors
3x Eye of Ugin
3x Crystal Vein
Vesuva can always copy an Eldrazi Temple if a Cloudpost isn't in play, or can even copy an Ancient Tomb if you're really desperate for mana. Thus, effectively, every single land above with the exception of Glimmerpost (this is why I cut it to 3) is capable of tapping for two mana by itself. This effectively lets the deck cut chaff like Mind Stones and Grim Monoliths and go very threat dense.
With a manabase as above, I would play no more than 10 Non Eldarazi cards at most.
I've been very pleased with the deck's performance, the curve is about as perfect as it could be.
I'm planning to take the same list with a sideboard identical to the list above to a Modern Tournament this weekend with the tweaks...
Both Sea Gate Wreckage and Conduit of Ruin + Ulamog proved to be a god send vs. Miracles. I think it's a mistake not to play them. Conduit has been so solid that I'm considering cutting a Revoker to play the 4th Conduit.
Sea Gate Wreckage is a far better way to combat counterspells than Cavern of Souls imo. The main cards that you don't want getting countered, Chalices and to a lesser extent Warping Wails and Revokers aren't protected by Cavern. In fact, Cavern makes it such that those key cards almost always end up being the ones that get countered. Wreckage on the other hand blows the game wide open and outright wins games all by itself if your opponents manages to stabilize while taking you to top deck mode.
Eye of Ugin is too explosive when paired with Mimics and Endless Ones not to play as a 4 of IMO. The occasional hands you have to mulligans are more than made up for by the hands that have you dropping down anywhere from 2-5 2/2s and 2/1s on your very first turn. Likewise, the acceleration that City of Traitors and Crystal Vein provides are well worth having to sac them in order to let you empty out your hand a full turn or two earlier.
Urborg effectively lets Ugin generate 3 mana, while also letting you use Ancient Tombs to make mana without costing you life.
Thoughts?
You're correct, that's the flaw with the approach outlined in the OP. There is an alternative however...
Playing a manabase similar to the below allows the deck to support both explosiveness and a long game with game ending threats...
4x Eldrazi Temple
4x Ancient Tomb
4x Cloudpost
2x Vesuva
3x Glimmerpost
3x City of Traitors
3x Eye of Ugin
3x Crystal Vein
Vesuva can always copy an Eldrazi Temple if a Cloudpost isn't in play, or can even copy an Ancient Tomb if you're really desperate for mana. Thus, effectively, every single land above with the exception of Glimmerpost (this is why I cut it to 3) is capable of tapping for two mana by itself. This effectively lets the deck cut chaff like Mind Stones and Grim Monoliths and go very threat dense.
With a manabase as above, I would play no more than 10 Non Eldarazi cards at most.
Here's the build I'm playing around with...
Here's my most current list as of 2/6/16
I've been extremely pleased with it's performance.
http://en.wikibooks.org/wiki/Magic:_The_Gathering
It's a wikipedia sister project, which means anyone can freely add to it, edit it etc. A couple of people have already contributed to it.
People can compile rumored cards there as well, discuss their pros and cons, etc. I figure it could work in unison with this site..