The Burning were a recognized threat to the whole of creation by the Guardians. Aside from their Gods, what do the Skrulls have to combat the Burning, let alone both the green and white martians?
Whites and Greens suffer something of a numerical disadvantage, but are individually still massively powerful. It depends somewhat on how fast the Skrulls can figure out their weakness and exploit it, because otherwise most fights with them are going to result in a lot of dead skrulls, often with little to show for it. Though strong enough forces will still be threats, as will be Super-Skrulls.
You do have fun stuff like Skrulls facing a species who can infiltrate them, but cannot be infiltrated in return (telepathy).
Burnings, as mention, are uber hax. They both reproduce faster and have no weakness, fire apparently strengthening them if anything.
The Skrulls stomps if we use what have been shown on panel. They basicall have the powers of every hero and villian on Earth and then they have a habit of mixing these abilities onto one Skrull. This fight isn't going to be pretty but the combination that the Skrulls have is outrageous.
Yeah there were some skrulls in the Secret Invasion that should not be triffled with. Plus they could possibly make Martian based Super Skrulls as well.
Super Skrulls are nice, but every martian tends to come with about as many abilities standard, and most of the super skrulls would IMO lose to an average martian.
Most S-Ss are outmatched by most martians.
And, non-Super Skrulls will really be screwed against Martians.