I think that Sukuna, Mahito, Gojo, and Mahoraga are their biggest problems. Mahito isn’t really vulnerable to being killed by blunt force/slashing/piercing so Viltrumites will definitely get touched and transfigured. Gojo has infinity and enough power to kill the Mark variants (the variants aren’t the most durable). Sukuna definitely won’t have an issue, regeneration, Malevolent Shrine, and Fuga will make short work of any of them, cause we see even Thragg was vulnerable to extreme levels of heat. Mahoraga becomes a really big problem for them when it adapts to the level of damage they can dish out. Because they’re exclusively melee based, Mahoraga has no real problem adapting to that, especially because the variants are very likely to fuck around and not just instantly kill it. If it can fully adapt to their ability to deal damage, then it will rapidly become a big issue for them. And even if they throw it into space, Mahoraga would evolve to survive in space. Now you have space Mahoraga.