Earlier quoted context omitted.
Yup, I unfortunately have to agree. It's a cool idea, but when I type "gimme" in the editor, the two adjacent m's are different widths, and it looks sooo wrong. Interestingly enough, the fact that letterforms change as I type doesn't bother me the way I thought it would -- turns out I'm already kinda used to it from ligatures. So points for being a clever idea to try out, but unfortunately what it does to a word like…
But is this a fundamentally bad idea, or is it, like kerning in general (which can require thousands of pairs to be defined), something that just needs more work? It's not like this is limited to just a few pairs of letters like 'mi', or just pairs - if you've seen Fontemon https://www.coderelay.io/fontemon.html you know the sky is the limit for what the rewrites can do!
1) Horizontal spacing needs to align vertically between glyphs of the same letters on separate lines. This is the example I give elsewhere of a letter "i" being in a very slightly different position than the same letter just two lines above it. I personally find this extremely disturbing and unpleasant to look at in monospace.
2) The glyph of a letter needs to be the exact same between instances of the same letter that are in any kind of visual proximity. This is the "gimme" example the parent of your comment gives. AnyThinG ElSe LoOkS lIkE jumblycase. Which I think we can all agree is just plain horrible.