IMHO He's not that powerful in the physical sense. But his knowledge in the mutant genepool makes him a very dangerous foe as he can make himself immune to most mutant's abilities.
Nate Grey attacked him simultaneously on all levels in the AOA saga but in the Onslaught saga, his counterpart found ways to counter X-man's power along with the rest of X-Force.
Nate Grey may be a loose cannon... but Sinister is "control"
ICD