EDTO rules are pretty specific, there are some traps to consider. A nice example I've personally seen was a flight to eastern China from Europe, there's a fair amount of high terrrain around the airport of Urumqi (which is usually a conventient intermediate alternate). That airport was closed by NOTAM, so the dispatchers planned an ETOPS segment with the ETP located very near to Urumqi. Here's the issue - its towards the end of the flight, and the diversion scenarios usually require more fuel than you have left at this point - so additional fuel is required. Now - because of the terrain, at least my company has escape routes that lead you out of this area, but those are much longer than the 'direct to' diversions which are considered for ETOPS/EDTO. You end up with a flight plan which barely accounts for 'direct to' diversions at FL100 to the next alternates, but in reality unfortunately there is a lot of granite in your way if you try to fly this.