Spoilers!
Like IT said, IT is powerful, but also not so much.
The truth is that IT only ever has as much power over a victim as that victim is willing to hand over to IT. That's why the cat targets vulnerable people at particularly low points in their lives -- as seen in the three interlude stories ITs victims tell you at certain points during the game. IT goes after people willing to give up control over their lives in exchange for the 'love' IT offered.
So while IT can warp reality and do everything shown in the different endings -- it's really only possible to affect a willing victim. Once a victim opens their self up to IT, there's usually no way for them to go back because by the time they've realized they were tricked -- IT has already absorbed them into itself.
IT isn't actually interested in doing anything like what's shown in the game. That was all just the protagonist's delusions getting warped and corrupted for fun by the cat. ITs only interest is to keep its chosen victims and make them part of itself.
Hope I explained that okay!