Even after looking up U+3164 (Hangul Filler), and reading (and rereading) the quoted paragraph:
The Hangul Filler character is used to introduce eight-byte Hangul composition sequences and to stand in for an absent element (usually an empty final) in such a sequence. Unicode includes the Wansung code Hangul Filler in the Hangul Compatibility Jamo block for round-trip compatibility, but uses its own system (with its own, differently used, filler characters) for composing Hangul. The KS X 1001 Hangul composition system is not used in Unicode, and the filler renders merely as an empty space; KS X 1001 composition sequences using modern jamo may be mapped to precomposed characters in Unicode. For round-trip compatibility, Unicode also includes the N-byte Hangul code Hangul Filler separately in the Halfwidth and Fullwidth Forms block, named the Halfwidth Hangul Filler.
I don’t think I’m any closer to getting it.
Even more importantly, I don’t get why it’s any more dangerous than the various unprintable special characters (like zero-width-joiner, for example). Is it just because it’s relatively obscure? Is it because it has a more complicated use (introducing, and optionally being a part of a Hangul composition sequence)?
It can be hidden in source code, for example to make a lookalike of a variable, there are many implications of that.
Hm, yeah, that’s why I thought of comparing it to the zero-width-joiner. However, what I want to know if there are dangers that are unique to that character.
