I haven't read the novel and barely remember the miniseries other then disliking it, but I have a huge issue with supernatural creatures and rules. I like when something has at least inferable rules like The Ring and isn't total inescapable bull shit like the 2016 Blair Witch movie or the Grudge.
Saying that I thought IT was explained fine. Knowing what the thing actually is, a really old and deep fucker type of evil that feeds on fear, it worked for me. It is powerful when It is luring a single person outside of a safe place, or a place naturally fearful like the dark basement or the sewers. The kids are always being lured away. If the kids are alone or don't feel safe, then they are vulnerable I think. When the kids are together or end up leaving, It backs off, presumably because it has all the time in the world (this thing is at least 200-300 years old, probably older) and It says the meat and flesh taste better the more fear is in it. Georgie and Patrick were alone and thus fucked. Although that doesn't seem to mean It wont try to attack them, like in the garage with the projector so I can see what you mean.
I think when it comes to mind trickery, a lot of things can be explained away. People seem to unknowingly enter some kind of pocket dimension that isn't perceivable by others, kind of like how Silent Hill works, which makes sense because King inspired the hell out of those games. I swear in the background of the library scene, the lady librarian is giving a barely noticeable death glare at the fat kid the entire time. Like you cant make out her face but its clear she is looking RIGHT at him. If you are able to break that illusion like when they open the garage door or leave the room then you are safe.
That's just how I read it.