things like the GHS hazard pictograms, DIN 4844-2, ISO 3864, TSCA marks, and similar such things seem like perfectly reasonable additions to Unicode
No they don't, because they are pictograms with very specific visual appearances. Such things don't belong in a character set, because things in a character set are characters. Glyphs (visual presentation of characters) live in fonts and each font designer is free to represent them differently, as long as they're recognisable. If every font has to represent things in the same way, then they don't belong in a character set, they belong in a set of standard images.
The other issue with this kind of cruft is collation. The unicode collation algorithm is insanely complex (and often a bottleneck for databases that need to keep strings sorted). Different locales sort things in different orders and most have well-defined rules for things that are characters. The rules for how you sort a dog-poop emoji relative to a GHS hazard pictogram, relative to a roman letter are... what?