Your last sentence is just wrong. Pantone encodes thousands of colors. RGB/CMYK can encode references to as many (within-gamut, but you can also hack around this) colors as you want, typically millions, often billions.
Sure, you might not get Pantone 123 if somebody asks for #ffc72b. But if somebody says "use Pantone colors only" and specifies #ffc72b, you're going to get Pantone 123.
Which of those components gives you how metallic it is? It's not even 1-1 as it depends on the paper used! Their own books show how lossy it is - they show the corresponding CMYK and it often barely matches.
The cardinality of one set is much bigger than the cardinality of the other one. Pantone 871 is mapped to #84754e. If you see #84754e and also get the instruction "use only Pantone colors", it is not ambiguous!
Sure, you might not get Pantone 123 if somebody asks for #ffc72b. But if somebody says "use Pantone colors only" and specifies #ffc72b, you're going to get Pantone 123.