I tend to think this is a bad idea. Most of the characters that can't already be displayed using fonts included in modern operating systems are likely to be characters from "obscure" scripts that may need complex shaping for correct/readable display, which won't work with Unifont because of its limited 1:1 character/glyph mapping, and/or they're characters (whether from historic scripts or newly-encoded emoji) in higher Unicode planes, meaning you'll need to provide not just the basic (plane-0) Unifont but also the extra resource for higher planes. It adds up to a lot of bloat, for the sake of inadequate rendering of characters that the user probably can't make sense of anyway.

The one case where there might be a worthwhile benefit would be for recent emoji additions. But that would be better addressed by a more limited effort to provide an up-to-date emoji-only font, not a resource that attempts (in vain!) to cover the whole of Unicode.