Lets say you come up with the greatest idea ever for a game which sadly needs a high poly morphing object for example.
All the engines you tried just sucked at this task because they didnt give you the access you need to transform your object and you had the right solution already worked out, just couldnt implement it, because it would mean to change the engine at some low level vertexbuffer management thingy.
UE4 gives you this kind of access.