"A complete consistency (either within the compass of the Silmarillion itself or between The Silmarillion and other published writings of my father's) is not to be looked for, and could only be achieved, if at all, at heavy and needless cost."
Christopher Tolkien
The Silmarillian Forward, 1977
It's similar to searching for consistencies amongst any mythology, which is what Tolkien was attempting to create. Tales will always change over time, and they'll always shift focus to what the teller determines is important. As focuses of a society shift, so do the focuses of its related mythology. In this way, I think Tolkien did an excellent job creating a united mythos for England in all the different versions of his legendarium. As the tales evolved, consistencies emerged elements which were formerly key, were discarded, and internal references became more commonplace than external references (see Tolkien's influences from William Morris and Icelandic, Celtic, Germanic, and Anglo Saxon epics)
That was the challenge Christopher noted in the forward to the Silmarillion. J.R.R. had started working it in 1917, and kept making changes right up until his death in 1973.
So he had 56 years worth of papers, and notes, lots of it hand written, to try to kind of reconcile into a single work.
I think he means in terms of time. In order to make it fully consistent, you'd have to have some kind of index and go through line by line making sure everything is saying the same thing.
I think he means in terms of the content you would have to discard. An idea is described ten times one way, but described once beautifully another way. To choose a consistent description is to “lose” the unique one.