Nair and dash attack could use a range debuff. I'd also recommend increasing the damage for the dash attack slightly. Neutral B needs some sort of debuff (Maybe non flinching lasers?).
Down B is a curious case. . . you can either make it less powerful OR you can see if there's a fix to the charging glitch. If there is, make it MORE powerful lol.
If I didn't make the glitches / techs clear in my last posts, here's step by step instructions on how to exploit these two moves.
Neutral B: As you initiate the move flick the control stick in your intended direction of travel. Once you get the timing down you can approach and retreat a ridiculous amount of distance. This can be canceled by going offstage during the slide which directly combos (possibly true combo, haven't experimented with escape DI) into fair, nair, or bair.
Down B: Short hop your charge animation. When you hit the ground you have freedom of movement WHILE you're still charging your move. (Be careful not to go offstage lol). This is kind of cool, but I'm betting it's unintentional. If you were to embrace the running charge, I'd recommend toning the power down a bit (because charging it is much easier to manage, maybe add on some electric or stun effects so you can use it in combos? I dunno, random suggestion). But if you fix the glitch I noticed the move took a while to kill against good DI, so maybe buff up the KBG a bit.
Anyway, yeah, those are my suggestions. I'd recommend some other players experiment with him further and see if their results mirror my crew's.
However. . . I'm pretty sure at this stage of development Tails counters Metaknight. We played that matchup a ton and Tails has answers to pretty much everything. Humorously enough, I think Captain Falcon might have a better matchup versus tails than MK does
