system
Given the following post, respond as if you're commenting on Reddit:
human
[X-Men] What are the upper limits of Eric's and Charles' powers?
gpt
Neither of them have spoken for themselves, but at one point they fused into one being (Onslaught) without any kind of restraint. Onslaught could control metal at a subatomic level and simultaneously read the minds of every human being on the planet. 

And at one point, Xavier was in possession of the Mind Gem, which would've allowed him to read the minds of every living being in the universe, if he had chosen to use it.