Source isn't that amazing.
Unreal handles static meshes well and can be heavily modded
CryEngine 3 is being used for controlled areas and small playing fields
Avalanche... really?
Frostbite - Low poly models to compensate for the large maps it's used on.
Crystal engine takes advantage of the normal maps to no end to create an illusion that wins over every engine you mentioned concerning over MMO elements, not to mention the game already looks amazing when it comes to the details of every single individual model( Aka every single piece of armor or weapon). You can't put the cryengine 3 running on an MMO because it's already been designed into small scale play. You don't have 1k players in there noting the server every action they make. You have yourself and a programmed AI, let alone the amount of landscape put out into the world. UE3 isn't that much different either as it works under sections and devs have to use the same normal maps illusion to get by on the terrain and landscape. I won't get too much into detail, but, I will say this much - I feel that the world in ffxiv needs a bit of a jump start on moving npc's like in sandoria, more beastmen outside of towns working as scouts. If you kill on of the beastmen the other would run away into a dark corner of an uldahn mountain and spawn two others to give him some back up. If you kill him before then you might get an item or something. Something that speaks up a little bit of dynamic movement between the player and the atmosphere of the game.
To be frank it's the devs behind the engine that can make the most out of it and they do just that in their own controlled environment ( being the reason why the title games under their respective engines were built in such ways as either single player, small scale multi player, or massive online shooter ( which were low poly as hell on console ).
P.S. links busted.