Sanmai impact on appearance & performance

Sanmai with high-carbon centre and folded sides is effective and attractive (if done well). My most expensive steel (in terms of $/inch) is done this way - kitchen knives. I think it's better than folding and differential hardening, but depending on the centre steel, can leave you with a more brittle edge (but harder). I'm not a fan of having visibly folded steel along the edge.

The visible difference between sanmai+folded and all-folded is that the folds disappear close to the edge. I like it, and prefer it over a gaudy (i.e., acid enhanced or polishing-enhanced) hamon.

Sanmai adds an extra step where things can go wrong, but the heat treatment can be simpler, saving a step where something can go wrong. Of course, you can differentially harden a sanmai blade.

In my experience, modern Chinese sanmai sword blades aren't super-hard, so aren't too brittle. One of mine suffered a little edge damage from hitting the steel support of my cutting stand, but was easy to fix.