I agree with all of your concerns of W3W. But one additional issue I have with it is that although remembering three words may be easier than a string of numbers, its too easy to confuse the words with other words. See below:
If you are lost and have to read these out over a radio, I doubt I could tell the difference between them! Even in a quiet room I may have trouble. You will have to spell it out, which at that point why not just give me your lat & long? Or MGRS, or Plus Codes?
I think Lat & Long, MGRS, and Plus Codes are better systems, and are open. Also there are paper maps with Lat & Longs, and MGRS available, so when tech goes down we are still okay.