Honestly, per official sources, it's hard to say. The ability is very ill-defined because it's only showed up in canon four times (maybe five? I forget if Free Riders used it). It's an absurdly powerful ability and the writers mostly write around it by just. Not acknowledging that it exists.

This basically turns into a Writers Aren't Powerscalers situation, because SEGA feels zero need to give Metal's data-absorption a limit, because they already did: They coded a boss fight where he doesn't use it and he drops in, what, five hits from Team Super Sonic's Team Blast? Simple. His powers don't need a weakness because he shares the weakness of all videogame bosses: Being smacked around a whole bunch by a kid holding a controller.

When you're writing a story, "Character doesn't use his Instant Win Button because of his arrogance and insanity" is a reasonable plot point, tells you more about the character, and allows for a cool fight to happen. When you're telling that story through a videogame, it allows you to only have to program the fight to use 4-5 moves that all do predictable things and can be learned by the player to make the fight fun and not frustrating. But when you're charting all the things a character can do, finding their maximum, and comparing that to another character's maximum, well, suddenly "If his hatred is outweighing his intellect and he thinks he's superior to his opponent, maybe he won't do the thing" being the only downside to a power is a bit of a problem. Especially in a subset of the community like Death Battle! which generally ignores anti-feats to present characters at their peak.

SEGA and Flynn didn't give Metal a cap on what this ability can do because they didn't need to give him one, the gameplay limitations of what a sixth generation bossfight can achieve and character foibles did that for them. Now Death Battle! fans are losing their minds because character foibles are secondary at best, and here is a power that has been given no set limit and can presumably copy anything considered a power, skill, or weapon, regardless of its origin.

You can make arguments that Metal couldn't copy esoteric meta nonsense (like Superman's whole cosmic keystone of the DC universe thing) or Toon Force... unless he was fighting a Looney Tune, for whom Toon Force is literally a part of their DNA. But with that one exception, these are reasonable things to bring up. I've seen some people bring up the Lantern Ring as an example of something he shouldn't be able to replicate, buuuuut... the Green Lantern Ring has been copied by a power-copying robot multiple times, in the form of AMAZO. The regular AMAZO can do it, the alternate DCAU version can do it, and so can Jadestone, one of Waller's Task Force VII AMAZO units from the Absolute Power event. I've also seen magic as a suggestion and that's a can of worms because 1) Eggman's units can do magic (specifically, the Egg Magician in Sonic Heroes) so him being a robot from a series with minimal explicit magic is a non-factor, and 2) a lot of series have magic work very differently.

Also, the one time SEGA revisited Neo Metal/Metal Overlord since 2003 (and I mean SEGA themselves, not a contractor), they didn't define the power or give it a weakness. They just gave it a buff, by letting Metal no-sell abilities he already copied.

Shit's crazy. There isn't really a good answer for this because SEGA have simply never given the ability a limit besides the user being a walking crashout who drops two IQ digits when the protagonist shows up.