which has he tested so far, and opinions on each?
The ones I've seen:
Shulk -> Ike-esque attack speed, monado arts a lot more counter-intuitive than expected. (they run out + can't just immediately switch after use. also: if you get hit while switching you get stuck on the one currently selected.)
Fox -> something about his lasers being useless, seems to have combo-ability.
Robin -> I think he personally didn't like Robin, but admitted Robin could be good?
Bowser -> Legitimately good.
Metaknight -> nerfed, low KO potential but might still be good. Zero said he might still main him.
He also tested ZSS, Peach, Samus and maybe one or two I forgot. I think for most of them his commentary was extremely move specific though.