When u look for Dangerous Unicode Characters for Testing

For your convenience,

U+3164 is ㅤ (unprintable), Hangul Filler, and U+537C is 卼, a Chinese character rarely ever used, meaning unsteady or dangerous.

Edit: I did not realize that U+537C may not be rendered on some devices without a CJK font installed. Here is what it's supposed to look like:

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)?

What does it do?

TIL that there are dangerous unicode characters..