Interesting idea. If, for some reason, your site goes down, will a note still be readable?
As any other platform WeExpire could temporarily go down, but it will be always back online before than the minimum inactivity time you can set up in your emergency note (1 day).
If you use this service (or any other of the ones mentioned in the other comments) you'll need to periodically check that they are ok (and that all your details are up to date).