I don't know that many will have had the time to play every single position, but I can give you my insights as I have tried several positions and learned a great deal about the mod.
I have won with the Japanese and I think they are very easy to win with. Simply hold what you have and go high culture in the home islands. For expansion, go south into Indonesia and Australia.
I think I have a good chance as the Russians in my current game. To meet the "colonize Siberia" requirements, you only need 3 cities there and there are plenty of reasons to go there. I have not started any wars, but had to fight the Turks when they came in and also had the Mongols attack me. I hold a good science lead and have a lot of things going economically for me. Overall, I would say the Russians are quite able to win their conditions. (I have to get to the Apollo mission and not lose a city until 1950 yet to complete and the harder part of that will most likely be the Apollo mission).
I have tried the Romans, US, Scandinavian, Turks, Chinese to name a few others.
The ones that I find really difficult are ones were the map is congested with a lot of other positions. I had the Romans rolling, but then Spain and France started popping on the map and I either had big fights I could likely not win or had to give up the cities. I was in somewhat of a spot like that with the Turks and had Egypt, Greece, part of Russia, Arabia, and parts of Germany under my control. The Egyptians came back to power along with a Greek revolt. I put down the Greeks, but let the Egyptians go as I did not have much military there. It was clear that the hand writing was on the wall as it was a matter of time before I ran into the same issue with the Arabians.
The issue I have with the mod (and it is really good, no mistake) is that when you conquer something, they really don't stay conquered. You will have to continue to fight with them over time in the game to keep them that way. Having them as a vassel state is not the same thing as running their cities and producing things for you. That and if you get a area taken care of, then there is always another civ that is coming in. (It sucks to play Spain because of this when Portugaul comes in).
For the tough ones, I would say the America positions (exception US) are very hard and anything in the middle east because you either have old civs that will rise up or have new civs that come in. China is hard as well. Mogolia, I don't see how you can raise all those cities and make the expansions you are supposed to. Any position that requires you to conquer a lot of land (Germany for instance) is tougher when you have other civs that pop up on that area.
Sorry that this is not a complete list of A-Z, but rather hopefully it will help you to better determine by looking at the conditions of what is tougher than others.
The best place to discuss stuff like this is Rhye's forum in the Creative&Customization section. In any case, I've heard that, for both Russia and Germany, it is a great idea to settle in Poland before the other, is that bottles them up...
I think that mongolia is deceptively hard. I've usually "conquered" china in a few turns. at least it shows that I've completed that task even though there are a lot of independent cities in what used to be china. I find with mongolia, I just can't expand enough. I think the next time I try that, I won't declare a state religon and research drama. I found I just can't build enough settlers to get the land grab in time. As for the previous mention of razing cities, I don't think there's a time limit on when you achieve that.
I've come pretty close to winning on germany ( just missed controlling russia). I think the important thing in rhye's is just being very focused on what the next UHV is. I found japan to be fairly easy though, although I took control of a new civ because I didn't find it too challenging. Rome I thought was fairly hard as well. with rome, the key is to grab the new cities that pop up, and holding onto them. also to take out carthage early. I haven't played the other civs too much, so I don't know how to rank them. of the ones I've played, I'd go