I think part of the struggle is that SourceMod doesn't exist in a vacuum; anything you do to the game may produce additional side effects that you weren't aware of, or may be ignored. GLua is different in that Garry's Mod is built around tooling to allow people to create things; SourceMod is bolted on to a black box and there's very relatively little functionality on hand to inspect the game state unless you get into disassembling the game itself.
The compiler thing is indeed a pain point - I'd love to see more user-friendly error messages, especially with the infamous "function prototypes do not match", but there's so few people familiar with the compiler to know what to change to make it all happen. I'm not sure of the state it's in now, but
I think it's worth opening a discussion for.
The introductory stuff on the wiki has a lot of good information, but it does take time to get to the meat of things. Silvers's guide, while comprehensive, is very much an collage of information without any direction. I've been working on
my own opinionated guide on and off for that reason, and I've contributed the latter half of
the Frequently Asked Questions section on this page.
As far as learning goes, the only thing I can really suggest is look at others' sources; and compile early, compile often. The former to get an understanding of usage, the latter to catch issues as they happen instead of getting bitten by something that you already stopped focusing on.
(Also, get used to refining your searches - while ideally you get what you're looking for, you should be getting zero results over completely unrelated things.)
All this information is annoyingly scattered all over the place, for sure.