Who are MM's worst/disadvantageous matchups?
I'm not Nu~, but having multiple opinions won't hurt, so I'll share
my current perspective backed up from watching, playing, and seeing what the other notable Mega's are thinking. Hopefully Nu~ can chime in if he has any insight. I already stated Sheik & Fox are bad MUs for him (both like -2 at least), but while difficult, it has been shown to be doable through Kameme's consistent prowess in Sheik and Peabnut among others do well against Fox players like Nakat. Both will still be a problem but I will say they still have room for optimization to where MegaMan will be able to keep up, especially if his punish game continues to improve. The rest, I all consider to be -1 at worst personally.
Mario is not nearly as fast as Sheik/Fox. Easier to keep out and play anti-grab, and Mega's disjointed aerials & tools can anti-air him or outrange as he tries to jump around the wall, although you have to be ready for it cause he has good airspeed. Ally has stated in the past he feels this MU is overrated in how badly Mario wins, especially after he has fought Smasher1001 multiple times. Not to mention Kameme beating Ally rather convincingly after he figured out what was holding him back on the last runback.
Pikachu is a tricky MU that you have to approach it a little differently but I think if a Mega recognizes the options for QA, they can play around him a bit easier overall. If the MegaMan is good at creating a bubble around himself as he moves around to deal with quick attack such as tools like Leaf Shield, Uairs to cover space and catch pika as he stretches his hurtbox (if he gets clipped his bad airspeed could mean trouble and will dissuade him trying to spam it), z-drop metal blades as a wall, nair, and disjointed bair/fair, he can handle himself quite well. Pikachu, in general, is reliant on that move in neutral because pellets > jolts and without QA, Pika is relatively fast but doesn't have much of any burst and his aerials can get outranged. The same issue applies with playing around Pikachu on the ground, he's decently quick but grab attempts can still get stuffed cause of kinda bad actual grab range. Scatt took Esam to Game 5 last time they fought, was a good visualization of the MU.
M2's speed, reflector, and shadow ball going through projectiles is a problem when combined on top of how hard M2 punishes any small mistakes in spacing. But on the flipside, M2's disadvantage and attributes allows Mega to ledge trap or kill M2 quite early too. Reflector can be baited out with good item play and M2 nair isn't free to beat pellets when we have our aerials to contest it. Smasher1001 & Peabnut from what I've seen are quite good at this MU and have fought WaDi and Abadango and put on a good showing.
Lucario, our gameplan fuels his neutral as he gains aura to kill us early and shoot aura sphere through our projectiles. But elsewhere we are quite good at juggling, playing around Lucario's grappler like nature, and edge-guarding him to where it's not as oppressive as you'd think. MegaMan can struggle to kill sometimes depending on the situation but he does have a lot of raw kill power in general when he can't land his setups like Metal Blade -> RAR Bair. I can see -1 but I doubt it'll be worse than that if so.
ZSS despite the current results at top level being in ZSS favor, I currently would consider even but I can see the arguments for -1. Proper pellet spacing are good at shutting down her short hop game and on the ground it'll beat paralyzer and make it harder to grab. We have good anti-airs in general and our low crouch in particular especially makes it really difficult for ZSS to land hits if we play it right. Scatt seems to be the best at this MU from what I've seen, ik he beat Luhtie in the past and has kept it close with the others.
Rosa is the last one that I consider having a
possibility for being a losing MU for MegaMan. This MU is tricky, especially with how well she ledge traps, edge-guards and juggles MegaMan once she gets going. But metal blade going through luma and still hitting, leafshield wearing being able to hit luma and not make it act while he pressures w/ blade in hand, crash bomb exploding on luma, and us being able to move and nair luma away helps us play around it. GP doesn't shut down Mega as hard as you'd think. Not to mention Rosa is light and very susceptible to MegaMan's edge-guards and bair in general once he can separate. Hard MU to learn but Scatt has beaten Dabuz in the past showing some merit in it being doable. Same with Peabnut beating Falln and trading sets w/ Eldin in his region.
Everyone else I think he can handle fine enough to where they are even at worst. It's mostly Sheik/Fox/Pika/Mario he has to watch out for moreso than the others as those 4 probably don't have a chance to move to even. There's a misconception that he loses to sword characters but it's quite the opposite actually; they all share a lot of the same weaknesses and it takes a specific pellet game to properly maneuver around them. Once you do it well you can punish them for endlag and whiffing, juggle them hard cause they all struggle to land, control the midrange better, force them to approach since they no projectile etc. Not easy to execute tho, especially when people jumping straight into sword swings w/ pellets just to get them sliced apart.